I just came across a collaborative math encyclopedia at planetmath.org
It is an open source encyclopedia with entries contributed by the user community -- rather like wikipedia, but specialized for math. It uses Latex for math symbols. It is for college-level math (and beyond), so would not be of much interest to powwowers.
But I did come across an interesting geopow:
Given 6 points along the circumference of a circle, ABCDEF, the points at which AD & FC, DB & CE and AE & FB cross are colinear. Why is this always true? For extra credit, prove the same result on a conic section in 3D. (Pascal supposedly got the extra credit part, but, like Fermat and many of us, the dog ate his proof before he could submit it to the MathForum.)