I've just started testing Ning. There still needs some enhancements, but it's a great place to socialize, learn and share. Great possibilities for the classroom or just to pursue professional development. I'll keep playing around to have a better perspective on where I want to head with it.
Just found this great site that I guess this is the place to keep it registered, "25 Web 2.0 Apps to Help you Learn". Nice collection of applications!