Invented on May 18th, 2007 (by Oleg Ivrii and Zavosh Amir-Khosravi) a "rigorous" defintion of CoolStuff is against our philosophy, so I will only give some guidelines what qualifies stuff to be cool:
Of course, classical university education has its merits too :-).
CoolStuff is now organized by Yuri Burda. Contact (in person, or by email) if you are interested in giving a talk.
Previous CoolStuff Seminars can be found at the following link: http://www.math.toronto.edu/oleg/css/coolstuff.html