I'm not really all that interested in playing sites that won't play nice with WeChall. I have a bias. I know.
I've got plenty to do on the existing linked sites anyway.
LordOric's link to the other Project Euler thread is broken. I think this is the one intended: Project Euler
I'd bet it is the rankings, not the T-Shirt, that make them call WeChall a 'contest' site. Still, I've increased my math/net/programming education at least 20x over since I started playing these sites, so even if WeChall is a 'contest' site the 'contest' vs. 'educational' distinction doesn't really make much sense to me.