EDIT: THIS is for steam indies.
Jamestown. Imo its the best shmup on steam, plus it has a good difficulty curve, its imo the best intrduction into shmups that noobs can play as the devs has designed the game so that it gives gamers a taste of whats so good about Japanese styled shmups without throwing them into the deep end like other shmups. (It might be lacking online co-op but its still a fantastic game)
Start out in normal mode (very easy) and work your way up to legendary (normal) difficulty. Then move onto gauntlet mode and aim for the 1 Credit Clear, also the challenge mode is worth doing inbetween the main game modes. (Just dont play with a mouse FFS)
Beat Hazard Ultra, I fantastic way to listen to your music
Aquaria, A fantastic mix of Echo the dophin, Odin Shpere and Metroidvania.
Recettear, If you dont like the cutesy Japanese art, then try to get past it because there is a really fantasic video game underneath it. Try out the demo.
VVVVVV I played this a couple of weeks ago, really fun and addictive platformer (try to beat Vendi Vechi) MUST BUY!
Amnesia, A horror FPS done right
Magicka and Torchlight, good diablo clones
Serious Sam HD Gold edition, while not an indie if you like PC FPS games the series (both the new and old versions) are a must buy
Also, Trine, Bittrip series, Crayon Physics, Zeno Clash, Darwinia, ARES, Shatter, Zombie Driver, Shadowgrounds, Mr Robot, Bullet Candy, Everyday Shooter, Killing Floor, Arma 2.
Plus keep your eye out on Carpe Fulgur (Reccetear) and Rockin Andriod as their localisations will be coming to steam soon, the localisations are from the doujin scene and there are some really fantastic 2D games that most western gamers dont have a clue about, make sure you buy those.
NONE Steam indies, Cave story, Splunky, Noitu Love 2, Hydorah, Blue Wish Resurrection, I wanna be the guy, Vanguard Princess, Gun Girl 2, all games by Kento Cho, Witch-bot Meglilo