I have $20 in Steam What should I buy?

buy 8 tf2 keys. and trade for a skyrim :D

Dragon Age: Origins

It might be better to wait for sales because the Ultimate Edition generally goes on sale for about $8.00

Terraria or Starbound if you're interested in sandbox gaming are both great values.

Dark Souls (w8 for sale it drops down to 75% off and is awesome), skyrim, EQN.

Aces Wild: Manic Brawling Action is supposed to be a sick beat-em-up style game with a combat engine the DBZ games would DIE to have in their games.

And when all else fails, Drunken Robot Pornography for the quirky FPS game, too.