This is a cool site that I tried when it was posted on fark.com a while ago. IIRC, it only streams for so long before stopping (I don't know how long, but less than a work day) and it only allows you to skip so many songs (found this out after trying to use it as a quick way to scan for new music - only let me skip something like 10 times).

I have found some interesting bands I hadn't heard of before - it seems to be excellent for that.