AnyWhere.FM is relatively new on the scene, I stumbled upon this online music player / organizer / community tool, which I'm finding to be fairly worthwhile. It's like Windows Media Player, but online. You upload your mp3s and can then play them from almost any computer in the world. Other users can play them too, but only in "radio-station" style from your playlists. That is, the song order within a playlist is random, and one can only skip 6 songs in a list, to stay within the licensing agreements. This is much like Live 365, a "be your own DJ" site, but a lot easier to use. Live 365 has more DJ tools, but for simple playlist publishing, it's easy.
The site is a little buggy, some of the flash controls quit working (e.g. column sorting) and you have to refresh the page. About once an hour, the site will refresh itself ("update") and you have to resume what you were doing.
I've been slowly working on the tags for my mp3s, but you can do so with this online player as well. Unfortunately, you can't download your songs, so your local copies can't me updated.
Adding playlists couldn't be easier, and adding *to* playlists is simple drag n drop.
And finally, there are community features, including a matching function, to see who else has a significant number of the same songs in their collections. You can browse the other users and listen to their playlists.