If you scroll down (or up if I have posted a lot lately), you will see a web widget for last.fm. I recently added this to share my music habits with the world. It tracks played music from both my ipod and my computer. I don’t know what I think of it yet.
Here’s an article reviewing all the main players in the Social Music space. I just found this, so I haven’t even read it to see if last.fm is what I should be using.