UCSF and U ofMaryland have one as well. Although so far, UCSF and UPENN are the only REAL supplemental app I've seen with essay questions which require thought.
The other ones only want extra info (ie. address, schools attended, work exp., etc.) These are all questions that werer ansewred on the AADSAS app.....makes you wonder if they even look at the AADSAS app.
I know so many people who do not apply to UPENN because of their secondary, it is ridiculous. Those three questions were very hard and one couldn't just bull them. I was fortunate the have found the secondary in May and I started working on it then, and it took me three or four solid weeks to refine my essays for it. But I think it paid off since I got that interview.
I hear that UPENN bases their interview questions right off your secondary questions. Which is a smart idea, I think.