There's an effortless new way to share iTunes libraries over the web. It's called Simplify.
The name is perfect because it's so easy. You only have to download a 4MB software package that takes less than a minute (available for either Mac or PC's of course) and it seems to really work like a charm. You send an invitation and/or accept one and 2-3 people can share the libraries simultaneously. Or you could could listen to your home music library at work. The whole thing has the Mac simple and elegant look, feel, and implementation!
I am using it as I'm posting this. My musician friend has a much more extensive library than I do, and it's just like Im listening to my own locally right inside of my own iTunes.
The service appears to relatively new, but there is another very favorable review of it here at the "StartupSquad" blog. You can't burn or download anything, just stream it, so it's totally legal. That means you can share DRM protected stuff too. Like pownce, this is another hot "trusted friend" kind of social web-app.