Note: this post is from 2006, outbound links may be broken.
Free Synth1 soundbank: ACK
ACK is aimed at ambient music and contains a wide range of leads, pads, textures, rythmic patches, SFX, basses, percussion and synth sounds: 128 presets in total.
Check this demo song of 10 minutes for a live recording of some 40 presets in this bank and this song was done entirely with sounds from the ACK soundbank. (Update 26-05-2008: Stephan’s mp3s were hosted on Panicnow, which has ended its service).
You can download ACK from Stephan Müsch’s website (free registration required).