BuddyPress.org

BuddyPress is a great idea if you want to migrate from Ning, though we might be a bit biased on this site
You can use this plugin http://teleogistic.net/2010/04/more-import-to-ning-goodness/ to help import your Ning users onto your BuddyPress site.

This site is obviously one such network. Some of my other favourites are:

http://irrationalgames.com/
http://www.h-mag.com/
http://rideoregonride.com/
http://thepioneerwoman.com/tasty-kitchen/