Thanks to the Appvent Calendar, Craneball Studios' fantastic blimp piloting game, Blimp - The Flying Adventures [App Store] (affectionately referred to as "that blimp game" around here) is free for the day. Featuring 20 missions, an awesome art style, as well as OpenFeint online leaderboards, the only acceptable reason to not download this game is if you already own it-- In which case downloading it again would just be silly.
You control your blimp using a mixture of on-screen buttons and tilting your phone to accomplish various goals like rescuing people or destroying enemy blimps. While I'm sure a lot of iPhone gamers either play with the sound muted or playing through the speaker, Blimp is one game you really should play with headphones on. It seems like just as much care went in to designing the music and sound effects as the rest of the game.
Blimp is a very fun game, and even has difficulty settings so you can still enjoy it, even if you're not very good at it. When it first came out, forum members went crazy over the game, and with it being free today you really need to drop what you're doing and get your Blimp on.