Assassin HTPC are selling off surplus OEM licenses for Windows 7 at a very good discount:
Windows 7 OEM:
Home Premium $50
Professional $60
Ultimate $70
(the prices are even lower if you are buying in bulk).
They provide instructions for downloading the actual ISO image, and they will e-mail you a copy of the license number, and send the actual COA sticker by USPS.
To the best of my knowledge they've been around for a while, but I've never dealt with them. But of you're looking for a cheap copy of Windows 7, this looks worth checking out.
http://assassinhtpc.com/products.php?product_id=26