The Windows 7 Beta 1 that was public-bound in mid-January has been leaked now, and you can get a copy on BitTorrent.
The beta expires July 1, 2009, and from what we read, it's a bit more stable than the versions people have been playing with for a few months. Hit the links below to get a copy, if you're OK with the fact that you're technically not supposed to have this just yet. [My Digital Life via BlogsDNA via Technovedad via Download Squad]
Update: ZDNet also has a look at the beta over here, and they concur that it's pretty good:
# This beta is of excellent quality. This is the kind of code that you could roll out and live with. Even the pre-betas were solid, but finally this beta feels like it's "done." This beta exceeds the quality of any other Microsoft OS beta that I've handled.