I'm finally getting to some old posts that I've been meaning to link to. CoreyH points to Audioscrobbler.
I've been using Audioscrobbler for a while now to suit my OCD music telemetry needs. It works as a plugin to whatever media player you use, Winamp 5 for me, and uploads every song you listen to for analysis at their site. They offer a host of features like creating a network of like users for you to browse and hopefully pick up new artists and such.
A very cool idea. The only limiting factor for me is that most of the music I listen to is on my iPod, not on my desktop or tablet. I'm not sure if the iPod itself keeps playlist history (or how it would handle skipped songs, because very often I will play all songs and skip individual songs to get more variety), but it would be very cool if someone wrote something to extract this and upload to Audioscrobbler.
Speaking of which, why hasn't someone already written an MTP provider for the iPod?