Poker Tracker is essential. I played without it for months, which is fine, but had no way of really analyzing my game. It's helped me fix some big leaks.

PAHUD is not as essential, but once you have it you realize how good it is to have. You go from saying to yourself "this guy seems like a bit of a loose aggro donk, I wonder if I should call" to "this idiot is 65/21 so I'm fine with felting TPTK here and hopefully stacking him"