As a consequence of involvement with one of our ventures, I have been exposed to lots of music sites lately. One with a really slick and stylish player is Anywhere FM. Funded by Y-Combinator, and headquartered in San Francisco you can upload your music collection to their site and then play it anywhere through their online player.
Similar to other services, you can also discover new music via friends or site recommendations. They also offer free music to registered users. To date they have over 3million tracks stored on their service.