Simplify Media purports to make your iTunes library available to you (or any of your ‘friends’) online. And it works with both Macs and PCs.
How well does it work? Not sure because I haven’t tried it, but if it works too well it might face a legal challenge. And if the online music service suffers death-by-litigation then perhaps someone say (paraphrasing Othello) that Simplify Media connected people with music –"not wisely, but too well."