Windows 7, $30:

[So_1337]

I felt this was information that was important enough to distribute. If you have a .edu email account and your college or university is enrolled, you can get Windows 7 Home Premium for $30. To find out if you're eligible and to have it send an email your way, enter your .edu email address on the following page. Enjoy!

P.S. Since I've had it asked, I dug through the FAQs to confirm that the offer can also be used to purchase Windows 7 Professional for the same price.

Through this offer, you can also get Windows 7 Professional for $29.99.

This is important, as Home Premium doesn't allow you to join a domain, but Professional does.