I have vague memories of promising to make a new Indie game thread in the modern FPS whine thread. The basic premise was that instead of being grumpy old grognards spending your rapidly decreasing lifespans cursing the menace of crappy overpriced AAA games that may or may not also be buggy/consolified and generally not for the audience you represent, take a gander at the burgeoning downloadable Indie market that represents some of the most innovative, fun and challenging games available today. Using my meager knowledge, I'll try to categorize the standouts and give a short description. Feel free to add discuss the games, and only the games. Most of these have demos as well, so no need to spend money blindly.
(I have not actually played a portion of these, especially outside of demos, so you're getting second-hand impressions and hearsay a lot of the time)
Strategy:
Frozen Synapse - A turn based tactical game, the battle system resembles the UFO: Afterlight series. Focus on scenarios and multiplayer. You get a free game to give away with every Steam purchase.
Gratuitous Space Battles - A diversion from the classical RTS, you design,equip your spaceships and give them basic commands and then watch the battle unfold without any further input.
Flotilla - A 3D turn based strategy where you control a small squad of space ships. The combat system is similar to the one used in Frozen Synapse with a timebar instead of strict turns. Randomized campaigns.
Space Pirates and Zombies - A more actiony space strategy where you equip, build and manage and small squad of ships in a giant randomized campaign. Technically still in beta, but the demo is available.
AI War: Fleet Command - A very large scale and technical RTS, gets pretty crazy/confusing at times.
Defcon - An awesome Wargames style global thermonuclear war simulator. Good for killing an hour in the office. Also really striking atmosphere once you get into it.
Darwinia - Crazy stuff, try the demo, I don't have a clue what this is about.
Harvest: Massive Encounter - Another large-scale RTS focusing on resource management and everything on a giant scale. Check the trailer.
Massive Assault - A relatively standard PC RTS, make of that what you will.
Star Ruler - A very complex 4X-style game with a giant procedurally generated galaxy and minute control over every little detail
Achron - The only RTS that uses time travel as an active game mechanic. Create paradoxes and acquire nosebleeds from just watching the trailers! Not out yet, but you can play the beta if you preorder.
APOX - Basically a squad based RTS where you have fewer units, but much greater control over them, Ground Control style.
Adventure:
Gravity Bone - A free concept game based on the Quake engine. Try it, it's free and only ~30 minutes long.
Machinarium - A beautiful hand-drawn point and click adventure.
Amnesia: The Dark Decent - The newest game from the Penumbra guys, amazing atmosphere and great horror.
Hacker Evolution - Hard to classify under anything, it's basically Uplink with some extra bells and whistles added on. There's also one sequel out and another coming soon.
Minecraft - A world simulator, you cut down trees, make houses, fight zombies, mine and do a billion other things.
Action:
Trine - Kind of like Lost Vikings, you have three characters who each have unique abilities that you need to advance, plus it looks pretty.
Osmos - A gravity-based arcade-style thing, try it.
Gish - An old platformer with some fun physics and style.
Terraria - Taking the Minecraft forumla, making it 2D and shifting the focus on combat and leveling.
Zeno Clash - Don't even ask, but the combat system is supposedly pretty awesome.
Puzzle
World of Goo - You build bridges and stuff out of gooballs. Very nice aesthetics and especially music.
SpaceChem - Never managed to figure it out, advanced puzzle game.
Audiosurf - Make your own puzzle game out of any mp3 you have. Basically an interactive visualizer.
RPG
Torchlight - ARPG from some of Diablo II's team. A little short and without multiplayer, but lots of fun.
Penny Arcade Adventures: On the Rain-Slick Precipice of Darkness - A hilarious RPG with an active-participation JRPG battle system, a Lovecraftian tilt and pretty good writing(if you like the Penny Arcade stuff). Unfortunately missing its last installment.