They may not drive the traffic of Stackoverflow, but from what I've seen most of them are great sources of information for their particular topics. There are several other Stackexchanges I personally have found quite useful (ubuntu, tex, cooking, gaming, and homebrew mainly). I also enjoy just reading popular questions and answers from various sites as they can be quite interesting. Some examples:

http://travel.stackexchange.com/questions/19941/one-foot-in-...

http://english.stackexchange.com/questions/1216/what-is-the-...

http://physics.stackexchange.com/questions/75459/why-do-chim...

http://skeptics.stackexchange.com/questions/17578/did-9-11-l...