DigiDrummer for iPhone
DigiDrummer is a fun little application that allows you to play your drums out on the iPhone. Using Digidrummer, you can play along your music or even record your own work, and play it back later.
The application has a very low latency. You can also choose from 14 different drumsets including my favorite METAL2. There are also 8 drumpads available (see above).
DigiDrummer 1.9 does come with overdubbing, which means you can record and overdub your plays. Overall, the application is great to kill time (especially if you are at an Airport waiting to catch your flight).