I use Poker Office with the table overlay, and it's a huge help especially when multitabling.
Stats such as Bet Flop % or Preflop Raise % can be very telling. When someone's Bet Flop % is extremely low, let's say 12 %, you check to them and they bet out, you can be very certain that they're not betting with thin air.
When somebody calls your flop bet and you see that their Flop-Turn percentage is 68% you really need not be worried about them having a hand. You're simply dealing with a notorious flop caller and can probably take down the pot by betting the turn (Turn-River % is very telling here).
There's a whole bunch of stats that can give you an edge in the game. And then of course collecting your own stats is a huge help when it comes to analyzing possible leaks and weaknesses in your game.