What are some must-have 2p <$20 games for PS3

I got a used PS3 from my cousin, and now I need to pick up some games for it. She included 2 games: Sonic Unleashed and COD: WaW. I bought Sonic & SEGA All-Stars Racing today. I also bought madden 09 and NBA Live 08 just because every now and then I like to play a sports game, and those were cheap enough.

I mostly play on PC for multiplayer, so the most multiplayer I'd be doing would involve me vs my girlfriend. So PS3, what would be the best cheap games to get? Which games are worth me spending some extra money on?

Thanks!