After you enter your email address (you can choose to subscribe to their newsletter too if you are interested) a page will be displayed with your registration codes for different platforms:
It seems that Download.com is sponsoring this giveaway (?):
For one day only, you can get an ad-free version of Opera. Simply click here to get a free registration code. This offer is valid from 12 a.m. Tuesday, August 30 to 12 a.m. Wednesday, August 31 2005 (PDT).
Thank you Opera!
Opera 10-year online anniversary party
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Opera Turns 10, Offers Ad Free Browsing