I suggest you go to the store page, then search from cheapest to highest, then scroll through reading all of the games up to $20. Though I can only really suggest saving for L4D2 off the top of my head.
Edit: You could buy like 4 $5 games with $20, I mean there are some awesome classics like Abe's Oddsey (Puzzle game) and stuffz.
Edit 2: Ohh there's Star Wars Jedi Knight Collection for $20.