In here, you will find some interesting links that you probably already know, but for those non-initiated, this cant be a great ressources spot..

STA league (http://tfc.stronger.org)

Stronger Than All. STA is a league with alot of different games like TFC. Most of the clans on it offer a great challenge.

OGL league (http://www.worldogl.org)

This is the most common league for any clan. Range from newbies clans to expert clans, you'll probably find someone with the same skills as you to fight against.