The
orDrumbox is a free drum machine software, designed to be as creative as possible with unusual features : auto-composition, polyrythmes, unusual arpeggiator, automatic sounds/track matching , custom softsynths, lowfi rendering, etc.
This tools can compose bass line and complete songs using included drum kits with the audio sequencer functions.
http://www.ordrumbox.com/download.php
Hydrogen (linux):
http://hydrogen.popez.org/hcms/node/21