TapDefense: You build towers that fire missiles that prevent demons from reaching heaven. A very addictive game that offer hours of fun. It is free and has provided me with hours of entertainment. Free with embedded ads
TapFarm: For those into farm management...well, have fun. Free. My only complaint, it connects to a game server to play. There is no reason for this and, with my iPod Touch, I have to have wifi access to play.
Plants V. Zombies: This is a great game that I'm thinking about introducing to my son. It costs a few bucks- $3 to be exact- but I think it is worth it.
Rock Band: Published by Rockstar Games, this is very similar in play to the full version, except you tap the notes (no guitar) and it only displays one instrument. You can nextwork two phones together for dual play. Try out the free version with one song, or the $6.99 version with actual tour opportunities.