Opera is celebrating their 10th anniversary and is offering free registrations for an add-free browser for today only.
I think you're supposed to email:
registerme@opera.com to get a free registration key, but I did and didn't receive a response quickly.
Someone on Slashdot posted an alternative that works. Go to the address below and it will generate a new registration key every time.
http://my.opera.com/community/party/...me%40email.com