Bandcamp try to make digital releases of your tracks, singles and albums as simple as possible. Free account.
Want to do remixes and need stems? Or just looking for acapella vocals to put music to? Look no further, ccMixter offers Creative Common licensed music.
Online notation program developed in Adobe Flex (require Flash player). Great interface. Playback works surprisingly well for proofing scores.
SoundCloud is intended as a tool for musicians, producers and A&R folks, to make is as effortless as possible to pass tracks (as well as stems) back and forth, in addition to browse and comment them. Free and paid accounts.