A recent thread at Moho forums by someone looking for programs to produce music produced a few good links.

http://myriad-online.com/en/index.htm
http://www.synthzone.com/softsyn.htm
http://www.freebyte.com/music/
http://www.seeingwithsound.com/javoice.htm
http://www.finalemusic.com/notepad/index.asp
http://myriad-online.com/en/index.htm

EDIT: Couple more sound/music related from Moho forum thread



VstSeq - Windows
http://www.kvraudio.com/get/1021.html

Krystal Audio engine - Windows
http://www.kvraudio.com/get/866.html

Synthfont - Windows
http://www.kvraudio.com/get/1030.html

Go here for shareware and freeware host and plug-ins.
http://www.kvraudio.com/