According to download.com:
For one day only, you can get an ad-free version of Opera. Simply e-mail firstname.lastname@example.org to obtain a registration code. This offer is valid from 12 a.m. Tuesday, August 30 to 12 a.m. Wednesday, August 31 2005 (PDT).
Now is a great time to fire off that email, it’s a great offer for a really good browser. There’s a rumor Mozilla will follow by providing a similar offer for an ad-free browser withing a given time period. It will be something you don’t want to miss.
Ok, Firefox is always ad free. But seriously, give it a go. Opera is a really good browser, especially for low end computers that don’t have much CPU. It’s fast and standards compliant.