Amazon Cloud Player
Amazon just rolled out a service called Amazon Cloud Player that lets you store 5GB of music free (MP3 and AAC). This can be upgraded to 20GB for free by buying an MP3 album from Amazon. Additionally, new Amazon MP3 purchases don’t count against your storage capacity. There is also a browser based player and one for Android, which I haven’t tried yet. Head over to Amazon’s home page for the press release. There is also a video describing the service.
Update: I just read that buying an MP3 album will only upgrade you to 20GB for a year. Also, Mashable has an interesting point about uploading illegally obtained music to the ACP.