Matchbook offers live betting for most of the games on the major networks: ESPN, ABC, TNT, etc. for the major US sports (football, basketball, and baseball)
Bodog offers live betting on things like what will happen on the next play, who will score next, etc, but the juice is pretty high.
Betfair (not open to US players for several years so I haven't played there since) offers live betting on pretty much every game imaginable, televised or not.