muse.net is in beta and looking like a pretty cool way to pool your collection of music into a digital library you can access anywhere. It's about convenience, not piracy by specifically not allowing sharing or downloading, just personal playback. I've got a desktop with most of my music, and a laptop with a smaller hard drive and only a subset of my collection currently. This looks like a good way to share a single collection between both, even though the machines are physically miles apart.
I added my muse collection under my currently listening header below.
[UPDATE: After coming home from the office, I picked up my powerbook and logged into my collection. iTunes popped up and began streaming mp3s right off my office desktop. Truly amazing.]