drumbit - A very easy to use drum machine.
Remixlive - Remixer, beat maker and finger drumming instrument for iOS.
Type Drummer - Drum by typing
Sampulator - Make (and record) beats on your keyboard
Musicca Online drum machine - Musicca provides a fully-featured Online drum machine that enables you to create unique rhythms.
Djay - djay gives you instant access to all your existing tracks and playlists from iTunes.