My first thought was, “what, Al Gore?”. After doing a little research, I can’t seem to find anyone else laying claim to inventing the infamous 125 x 125. So, the crown goes to Nik Cubrilovic who was with TechCrunch at the time. Via Uncrunched:
Nik Cubrilovic had “invented” the notion of a 125 pixel square ad that we’d sell for a flat rate, which was an unqualified success and was quickly copied by everyone else.
It will be interesting to see how long the 125 x 125 hangs around. Whenever I see an 88 x 31, I immediately assume the website has not been updated in about 10 years. The 125 x 125 still has plenty of time left, but it’s definitely becoming less and less “preferred” by advertisers.