Free Audio/MIDI Sequencers, Editors, Notation software

Digidesign - ProTools Free
http://www.digidesign.com/ptfree/
- probably the best free Audio/MIDI sequencer available
- it works with almost every sound card (does not require TDM hardware)
- limited to 8 tracks of audio and 48 tracks of MIDI
- included RTAS plugins: EQ, Dynamics, DirectConnect, Mod Delay, Dither
- sessions are fully compatible to "large" Pro Tools TDM systems
for Windows 98/ME and MacOS 9.x (no Win2000/XP)

FruityLoops
http://www.fruityloops.com
- excellent Loop Creation Tool and Step/Pattern Sequencer
- very good for: Dance, Trance, HipHop, Drum'n Bass, Electronica, ChillOut, etc.
- matrix editor included in full version
- demo has all features except saving, but it can export the song as WAV or MP3 files
- comes with a wide range of samples, loops, synths, effects and demo songs
- very cool interface, but very different from Cubase and Logic
for all Windows versions (98/ME/2000/XP)

Sony Media Software
http://mediasoftware.sonypictures.com/download/freestuff.asp
- free loop and sample arrange tool: ACID Xpress (with realtime stretching/pitch shifting)
for Windows 98/ME/2000/XP

Finale Notepad
http://www.finalemusic.com/downloads
http://www.klemm-music.de/coda/notepad/index.html (German version)
- many features missing, but compatible with the full version of Finale
for all Windows versions and MacOS X

ReNoise
http://www.renoise.com
- Audio/MIDI Tracker
for all Windows versions

LAoE
http://www.oli4.ch/laoe/
- free Graphical Audio Sample Editor
for all Windows versions (98/ME/2000/XP)

Prosoniq
http://www.prosoniq.net
- free audio editing and DSP processing: sonicWORX Basic
(for MacOS 9.x)