I'm involved in some league play through
http://www.worldamateurpokertour.com
I personally really enjoy league play, and there are cash prizes available. Plus, they runs both leagues and sponsored tournaments. Members only, so very limited participation. No 6000 person tourneys. WAPT started off as a small thing based out of a free play site. They have really started to grow. The basic membership is free (although there is a different one available that you can pay for too with more benefits).
There are a lot of great people there and plenty more that I could say, but the best thing you can do is check out the website. I've looked at some of the other league availability out there, but I really like this one for the simplicity and the price. The other choices out there may have one perk here or there that is better than WAPT, but over all WAPT wins across the board.