Ardour is a digital audio workstation that you can use to record, edit and mix multi-track audio. It also allows you to produce CDs and mix video soundtracks, and it's great for testing out new ideas about music and sound. (open source)

Audacity is open source software for recording and editing sounds. It is a solid cross-platform sound editor supporting mulitple formats with an intuitive graphic user interface. (open source)

CDex is a tool that enables you to extract digital data directly from an Audio CD into plain WAV files (useful for making compilation audio CDs) or compressed using an audio encoder. (open source)

Hydrogen is an advanced drum machine for GNU/Linux. It's main goal is to bring professional yet simple and intuitive pattern-based drum programming. (open source)

Juice is the premier podcast receiver, allowing users to capture and listen to podcasts anytime and create their own audio channels on the fly. Highly recommended application. (open source)

MP3myMP3 Recorder saves any audio you hear on your computer straight to mp3 or wav. IF YOU CAN HEAR IT, YOU CAN RECORD IT! (open source)

MusikCube works well as an alternative to iTunes to manage your music collection. " It helps you just listen to what you want to hear,while offering innovative features not seen elsewhere." (open source)

Rosegarden is a professional audio and MIDI sequencer, score editor, and general-purpose music composition and editing environment. This is a strong contender when it comes to working with Midi. (open source)

Traverso is a cross platform multitrack audio recording and editing suite, with an innovative User Interface. Its a complete solution from recording to CD mastering. (open source)