For Windows ...
http://www.go2keys.com/
Based on the good things said about go2keys.com, I actually went for it and bought a Windows 7 pro key from them, having got the .iso file from the Microsoft Technet pages. I have been using it for just under a week now and no problems. On the system page it has the Genuine Microsoft Software logo. All in all I'm pretty happy and would recommend it to anyone. Put simply, it's legit!
more info;
http://answers.yahoo.com/question/in...8224944AAgkZoq