Xoscillo is free oscilloscope software that acquires data using an arduino (with custom firmware) or a Parallax USB Oscilloscope. Aguaviva is developing this project to support more platform.

Rosegarden is a well-rounded audio and MIDI sequencer, score editor, and general-purpose music composition and editing environment.
Rosegarden is an easy-to-learn, attractive application that runs on Linux, ideal for composers, musicians, music students, and small studio or home recording environments.

MuseScore is a free cross-platform WYSIWYG music notation program that offers a cost-effective alternative to commercial programs such as Sibelius and Finale.
You can print beautifully engraved sheet music or save it as PDF or MIDI file.

Renoise has a unique bottom-up approach to music making. With its vertical timeline and streamlined interface, Renoise gives you direct control over the composition.

DeVeDe is a program to create video DVDs and CDs (VCD, sVCD or CVD), suitables for home players, from any number of video files, in any of the formats supported by Mplayer. The big advantage over other utilites is that it only needs Mplayer, Mencoder, DVDAuthor, VCDImager and MKisofs (well, and Python, PyGTK and PyGlade), so its dependencies are really small.