I would have assumed it would be the ad companies hired by Renderosity.

About 10 years ago, Gamefaqs had some problems with that.  It wasn't popups, but there were some very intrusive ads, including one with a bug, and two that were indecent (one was Jamster/Crazy Frog-- they put a Norway maple leaf on him, but it didn't cover most of the naughty bit.  One of the Canadian forumites was offended because he mistook it for a sugar maple leaf.  It was also really annoying because he spastically jumped back and forth.  The other one was a Drawn Together ad the censored version originally aired on Comedy Central, and was still pretty risque of one of the characters playing grabass with another)  The buggy ad blared a really crappy guitar riff really loud-- it was supposed to be muted by default but was instead the other way around.  Then there was the dreaded "Coke ad", which made noise every time your cursor ran over it.  Oh, and popovers.

 

The indecent ads BTW actually violated CNET's ToU, and were taken down usually within 24 to 36 hours, but still it really tells you something.  Granted, this IS Cnet we're talking about.

 

If you determine it to be the handiwork of the advertisers, then a complaint to the ad company is in order, as chances are popups violate their ToU.