I know that it's been online unofficially in several places, but I have good news if you're wanting the official Windows 7 BETA. It's being released on Friday. But, if you have a TechNet or MSDN subscription, you can get it right now.
From what I've heard, it's a great experience so far and runs better than XP in some cases. I can't wait until friday comes so I can download it and try it for myself.
EDIT:
Windows 7 BETA x86 ISO
Windows 7 BETA x64 ISO
Go HERE to get your key and image for the beta.