LMMS is an open-source and libre DAW (digital audio workstation) that's nice and easy to use (comparatively, at least).

I haven't really dug into using my music gear with LMMS outside of the little keyboard I have, but here're some links:

• Post your MIDI-Hardware that works in LMMS here - LMMS Forum thread.
• List of working midi keyboards on the LMMS wiki.

• LMMS