I like the 125x125 image ad format. It's just big enough to get my attention, and small enough so the publisher can stack in a few of them without taking up too much space. I think it's been gaining a lot of popularity lately for those who sell adspace directly to businesses.

A few examples that come to mind

http://betterexplained.com/

http://theoatmeal.com/comics/coffee (bottom of the page)

http://37signals.com/svn/