I recall there being a thread devoted to a rather simple opt-out option for pop unders. It's something that I ran on a previous computer, but that computer is now spare parts, and I'd love to run it on this one.
However, I can't seem to find said thread. I've done a Google search on The-W. I've perused the Internet & Computer forum here. I've even dug through CRZ's World, because I think he mentioned it after it had been around awhile.
According to C|Net, Opera will be offering a free registration code for one day only. The note at the bottom of the Opera descroption states "This offer is valid from 12 a.m. Tuesday, August 30 to 12 a.m. Wednesday, August 31 2005 (PDT).