I've been on a 40K kick for a few years so that's what most of the functional armies tend to be:
Dark Eldar: This is the army I play the most and am most current on. On day I'll actually build that big Slave Barge I keep buggin' Forgeworld to make... Ok Forge World did finally put out the Tantalus. Its not technically the slave barge of my dreams but its awesome and Raging Heroes added some delicious banner bearers so I'm pretty happy. *UPDATE* With Raging Heroes doing a full blown army of evil space elves in The Toughest Girls in the Galaxy 2 (TGG2) I'll add a lot of variety to my main force... especially some delicious new, and badly needed, Grotesques... really grotesque!
Orks: My favorite army to tinker with and I even play it now and then. Its loaded up with oddball stuff (as befits the Orks!) including a half dozen super heavies and Gargantuan creatures and an evolving all Gretchin mercenary company lead by the infamous Zoggit on board 'Grotzilla!' This Grot Mega Tank heads up a ramshackle squadron of tiny grot tanks and a horde of makeshift grot buggie/trukks? Zoom Zoom!
Tyranids: This is an army that my wife loves to play so it gets a lot of attention. Its probably the largest of the armies in raw figure count but the Craftworld Eldar are pretty close. There is a load of new stuffs for these guys.... ack!
Sisters of Battle: I'm developing this as an Urban Assault Force and am in the middle of construction for a fortress/chapel/security forces complex. This facility will house Witch Hunters in general and may also serve as a fortified Precinct House for Adeptus Arbites. *UPDATE* I went insane during the Raging Heroes Kickstarter(TGG1) and now am ramping up the HSF (Heresy Suppression Force) into a full blown army in its own right. Format still sorting but I'm seeing them as Ecclesiastic Regulars to perform Police and Imperial Guard functions for the Ecclesiarchy. UPDATE: The Raging Heroes TGG1 Kickstarter now forms the basis of three full blown armies. The Kurganova storm troopers (KST) will continue in support of the Ecclesiarchy (In spite of their closely guarded 'family 'curse' of lycanthropy). The rebellious Jailbirds will often form allied "Frateris Militia" or conscripts in KST service but also form an independent Light Infantry formation in opposition to the KST oppression.... yummy! The Iron Empire is shaping up as a Proto-Necron force with some fluff developing them as the reclusive and super secret remnant of the race that ultimately became the Necrons. The Sisters of Battle themselves will be bolstered in 2015/16 by the arrival of the Raging Heroes Sisters of Eternal Mercy. I think I'll continue the GW sisters as the Order of Our Martyred Lady (and Elements of The Ebon Chalice) and ally the Sisters of Eternal Mercy as a daughter organization... heretical...perhaps!
Dark Angels: This is an art and demo game project. Filled with crazy modeling goodness. I took a display from this force to Gamesday 2011 to enter Project Thunderhawk into the last chance qualifier for Armies on Parade. It was fun giving folks a look at the original all metal Thunderhawk complete with electronics and its own hangar. I'm not done with that project yet so I expect to revisit it and upgrade the paint as my vehicle techniques improve.
Craftworld Eldar: This is a legacy army that is just enormous, even by the scale of my normal armies, and is due to get a major update with the addition of a pile of Forgeworld models that I've been sitting on for about 10-15 years as well as a ton of new stuff.
Chaos Demons : Chaos is something that I've wanted to build and always put off until "I was a better painter" I decided that, with my eyesight slipping, I wasn't going to wait and painted up an apocalypse sized force of mostly Khornate Daemons. My real chaos love is Nurgle but my rule of thumb is "paint whatever is open" I got pile of Khorne from a guy and it was all out of the blister so... Poof! "Blood for the Blood God!"
Chaos Space Marines: This is an army that I've collected and prepared for but have painted in fits and bursts. Its in need of a total review to make it more coherent to look at.
*UPDATE* I've salvaged a big pile of Nurgle based CSMs and done a lot of painting and some fluff work on the "Siwa Contagion". Which is a Warband associated with "Unwritten" a Tzeentch flavored Warband lead by the "Oracle of Siwa". How these two became associated must be an epic tale given the notorious animosity between their patron deities.
Harlequins: The servants of The Laughing God were actually my first 40K army back in the bad old Rogue Trader days. They've recently returned as a full blown army.
Ynnari: Because having 1.5 versions of "Evil Space Elves" doesn't seem to be enough.
Warhammer Fantasy Battle:Fantasy Battle is what I played the most for many years. It has been neglected in recent history but could be dragged out at any moment. UPDATE: The End Times are upon us! What an exciting, and scary, time in the Old World! Up-Update! Age of Sigmar has arrived but I still seem to be setting everything up as WHFB armies. In truth I most loved Third Edition WHFB and that is probably what we'll end up playing the most at the house.
Vampire Counts: I began my journey as an Undead player at the tail end of Second Edition Warhammer. The Undead were my primary army until it was broken up after 5th edition. I've slowly reformatted both of the undead armies to be stand alone and functional. UPDATE: Now that Nagash has returned the Undead have come full circle! My old combined Undead Legion is once again a real army in Fantasy Battle... complete with amazing new hard plastic models! UP-Update I'm poking around the Alliance Death for Age of Sigmar and, if I play AoS, it'll probably start with the undead.
Ogre Kingdoms: This a good sized army painted to a higher standard than most of my armies. There is a lot of personality and... with the new codex and models imminent, may get a visit from my paintbrush again soon!*UPDATE* Some characters have been added recently. Greasus Goldtooth, Skrag the Slaughterer, and an awkward metal Scrap Launcher.
Tomb Kings: I'm currently kinda stoked about this army so may be working on it again!
Slann: Yeah I know they are dead and have been replaced by Lizardmen but I do have an enormous Slann Army that I still model and tinker with... it still has a horde of pygmies! shhh... don't tell anyone...
Lizardmen: See Slann above! Its coming along...
Wood Elves: My wife's preferred fantasy army and a real joy to model and paint.
Chaos: I know this has been broken up but I'm tinkering with all the flavors...
Orcs & Goblins: How could I have forgotten the Waaagh! Actually there are a lot of gobbos in there... look at all the wind up toys!
Skaven: I like the rattys! This is a small army that I paint on from time to time while mumbling paranoid nonsense...
Dark Elves: This is an incoming army. A mix of Games Workshop, Raging Heroes, and Avatars of War models. Its going to be a foray into hedonism.
Sisters of Eternal Mercy: While not an official army in WHFB It will be a fantasy battle equivalent of the 40k Sisters of Battle. Probably an Empire based female militant order rising to importance during the End Times due to the tremendous loss of fighting men. It'll be composed of Raging Heroes minis in the main and might double as Basileans for Kings of War.
Dwarf: Bevie has discovered her inner Dwarf and has commanded a Throng be raised for her use! This army will be a mix of Mantic, Avatars of War, Atlantis, and Games Workshop minis. I'll also use it for other theings like Kings of War or Warthrone.
Chaos Dwarf: I ran a Chaos Dwarf allied contingent in Third Ed WHFB and this is growing to a full blown Army. Its a mix of GW, Mantic, and Forge World.
Escher: Because I like painting chicks!
Goliath: Because I had them.
Redemptionists: On fire!
Diggas: Because Gorka Morka is really Necromunda!
Sisters of Sigmar: The first Warband I saw in the big box-o-Mordheim. Full sized now with some available personalities to hire for support.
Witch Hunters: Because its the closest faction to the theme of the gorgeous Raging Heroes fantasy Women... and a fun theme to paint.
Tabletop World: While not a faction these buildings will likely show up in every game of Mordheim. They are scaled for 20mm square bases so are a bit of a tight fit for games that feature the more recent, and larger, round lipped bases. Therefore... it belongs here!
Eldar: Because they look cool.
Imperial: Because there are so many.
Chaos: Because there are so many and they look cool.
Flames of War:
2nd SS Panzer Division "DAS REICH": Because there was a boxed starter army available. I've since built on to it and its a respectable force.
Colette Dubois: Girls gotta dance!
Rasputina: Nothing is as cool as an Ice Queen!
Kirai Ankuko: Spirits of Vengeance. Super Japanese angry spirits! This is my primary faction of the moment.
Guild: Mostly because the new hard plastics of lady Justice and the Death Marshals are too cool not to paint!
Kings of War:
Abyssal Dwarves Because Games Workshop punted The Old World! I'll build these for both systems... maybe even adding in some Forge World goodies... shhh... it's a secret!
Dwarves It began with Berserker Brock riders and themed movement trays.... now its a monster of it's own.
Undead: I just can't escape the moldy bones!
Basileans Sisters of Eternal Mercy... angels and all!
Veermyn Because Games Workshop never did Rats in Spaaaace! Space Skaven FTW!
Forge Fathers Because Games Workshop exterminated the Squats.
The Plague: Because this game is becoming much cooler than I thought it would be! The Plague are more complex than they might appear at first glance.
Everything Because there are only two factions at the moment and I love the look of the ships.
Secret Agents Ok I got this because the minis are gorgeous... I'll play the game... and hope its good, but in the meantime... these will double as Imperial Agents and spy NPCs for several other games.
TBD Like I really need another sci-fi mini game... but I've admired these for years and I like the idea of a less Gothic space game.
Vatican: Alternative timeline Vatican troops
Strigoi Vampires for the Vatican to oppress/be oppressed by.
The Wolfen of Ylliaa: Really great Wolf minis... Arrroooooo!
The Ophidian Alliance: Monstrous and serpentine!
TheCynwall Elves: An elf for every game!
A boutique miniature manufacturer with an as yet unreleased game system for which I've collected every faction. Stay tuned for an update to this very soon!
*The Toughest Girls of the Galaxy Game goes to Kickstarter later this year!
The Dickensians: Who doesn't love some malignant magically infused steampunk urchins?
The Servants of The Engine: Hi class bad guys and really cool steampunk mechanicals!
The Guild of Harmony: They have the Monkey King! 'nuff said!
The Egyptians: I too fell for Indiana Jane.
That is most of the coherent stuff. It represents the current state of affairs and does not include my vast amount of Micro Armour (Modern and WWII), the many fleets of ships from all eras, the DBA binge armies (Mostly ancient and Medieval), Warfare in the Age of Reason Marlburian French, Various Warhammer fantasy fleets, The Empire of the Petal Throne (Qadardalikoi and Missum rules) etc...