Audio Jungle complied a nice list of 29 Linux Music making Apps. The better known ones like Ardour, Audacity, and Rosegarden. Many other are included also that look worth a try. Glancing over the list most of Open Source or at least free with at least one exception the commerial trakcer Renoise.
Here is a bunch of cool stuff I have found lately:
The Openstomp Coyote1 is a product seemingly made for me. A stomp box that you can program to be any thing you wish. Sound great but it is not yet released. I am going to keep an eye on [...]
Intua are planning on releasing a music making application for the iPhone and iPod Touch. It looks slick as would expect for anything Apple handheld related. Here is the feature list from the site:
16 triggerable pads with polyphonic control
Automatic sample slicing on multiple pads
Edit pads individually
Duration [...]