I’ve been playing around with Hydrogen – an advanced drum machine for GNU/Linux. So far, it seems to be full featured (I’m more familiar with “real” drums). Hydrogen can import sound samples in .wav, .au, and .aiff formats and export “songs” to midi and wav files. Hydrogen uses an XML file for its native song file format, thus allowing the potential for manual tweaking of song files and the ability to manipulate these files by other applications.
Hydrogen might just have the best splash screen ever: