Here in Philadelphia we're about to experience a pretty fascinating wide open mayor's race. Lots of contenders, and nobody's putting any money on who's going to win. Just-resigned city councilman (the law says you can't hold the office and run for mayor simultaneously) Michael Nutter become the first to declare, and any more are expected to do the same.
For those who are interested in following it, the best resource I'm aware of is The Next Mayor, a joint project of the Committee of 70, WHYY, and the PDN.