Ardour is a digital audio workstation. You can use it to record, edit and mix multi-track audio. (GPL)

Audacity is free, open source software for recording and editing sounds. (GPL)

Jokosher is a free, non-linear multi-track audio editor. (GPL)

LMMS aims to be a free alternative to popular programs like FruityLoops/FL Studio, Cubase and Logic allowing you to produce music with your computer. (GPL)

MusE is a MIDI/Audio sequencer with recording and editing capabilities written by Werner Schweer. MusE aims to be a complete multitrack virtual studio for Linux. (GPL)

Qtractor is an Audio/MIDI multi-track sequencer application written in C++ around the Qt4 toolkit. (GPL)

Rosegarden is a professional audio and MIDI sequencer, score editor, and general-purpose music composition and editing environment. (GPL)

Traverso is a complete solution from recording to CD Mastering. (GPL)