Is this similar to what My.MP3.com or MyPlay used to offer?
The only similarity is that like those services Muse.Net is all about ubiquitous access to your music. Those services stored your music on their servers, Muse.Net does not. Muse.Net allows playback to remote machines, those services did not. Muse.Net offers a variety of interfaces along with APIs and SDKs for developers to easily create new interfaces from. Muse.Net offers album covers, artist bios album reviews and soon other contextual information to accompany your music. Muse.Net will soon start to offer community features surrounding your collections. Muse.Net virtualizes your collection and provides an API to do all sorts of rad things with that collection. It is not merely a web site.