Again: VVVVVV (http://store.steampowered.com/app/70300/?snr=1_5_9__13) - $2.50: I cannot recommend this game highly enough. It's as good as, if not better than, the best metroidvania games, but entirely puzzle and exploration driven. It's challenging yet not frustrating. (The beginning is boring but after you find the first crew member it becomes utterly mesmerizing.) Easily the best $2.50 spent; there's a demo of it, too. Hell, even if you don't love it to no end, it's only $2.50; you can do a lot worse and very little better for $2.50. Tip: M-key mutes sound.
Madballs in...Bobo: Invasion (http://store.steampowered.com/app/25700/) - $2.49: A good top-down shooter. The single-player is entertaining for the first run (unfortunately iirc you must beat it a second time to unlock all multi-player options). However, the multi-player is a blast but I think it's pretty much dead. Haven't played it in a while. I have an additional copy if anybody is interested in it.
Galcon Fusion (http://store.steampowered.com/app/44200/) - $3.40: Utterly fantastic. It's a simple, fun, and frenetic RTS. For $3.40 it is a steal. Highly recommended, even if you suck at strategies. This game is entertaining for an afternoon of gaming and unrivaled five minutes of gaming.
Magicka (http://store.steampowered.com/sale/MagickaSale) - $3.39; Complete (all DLC) - $8.49: Inventive, funny, and most importantly fun! The complete pack is totally worth it.
Unreal Tournament GOTY (http://store.steampowered.com/app/13240/?snr=1_4_4__13) - $4.99: You owe it to yourself to play this game, or at least 2K4. Arguably the best arena-shooter ever made. This is a shooter in which skill and reflexes take precedent to grenade-spamming and luck. After you learn the weapons and movements, it's pure skill.
Unreal Tournament 2004 (http://store.steampowered.com/app/13230/) - $7.49: The differences (primarily mods) between this and GOTY are enough make both worth the purchase, but if you're on a budget you might get more mileage out of this one: active enough community (though I think GOTY's might be more active), prettier, more mods (I think), and it should run fine on any modern system.
Torchlight (http://store.steampowered.com/app/41500/?snr=1_5_9__13) - $7.49: Diablo in disguise? Maybe so, but it's still a fantastic hack-n-slash. There's no multi-player. Yeah, weird, but it's still a very fun single-player affair.
Super Meat Boy (http://store.steampowered.com/app/40800/) - $11.24: As fun as it is tough, and it's relentlessly tough. Its trial-and-error game-play borders the absurd at times, though its charm and simplicity make you feel all warm and fuzzy inside. Do you enjoy those intense moments during which you involuntarily bite your lips, clench your jaw, and contract your anus? Yes? Then you'll love this game.
Overlord (http://store.steampowered.com/app/11450/) - $3.34; Complete Pack (1+exp+2) - $13.39: A good haphazard mix of Pikmin and Zelda. Overlord is mostly better than Overlord II, but they're both excellent. Raising Hell contains new dungeons.
Frozen Synapse (http://store.steampowered.com/app/98200/) - $16.74: (only a two-pack is available.) Easily my favorite TBS, and a GOTY-contender without a doubt. Turns are taken simultaneously and your plans are transformed into five seconds of real time. With as much control other your individual units as you'd find in a typical FPS, the game truly affords you complete control over every movement your units make. Surprisingly, though, is how simple it is to control your units. Every action is initiated through a simple way-point system (think C&C) and context menu; no overwhelming GUI to learn! It is the equilibrium of simplicity and complexity.
The Witcher 2 (http://store.steampowered.com/app/20920/) - $33.49: You really should get it.
But above all else, VVVVVV (http://store.steampowered.com/app/70300/?snr=1_5_9__13).