Google Analytics. On the plus side, it's free and easy to implement. On the negative side, I don't like the way it handles referrers.
The downside for with GA is the non-realtime status. Sometimes takes more then 12 hours for the hits to come through in the dashboard. Other then that I really like it. I think Mixpanel does do realtime, but that single feature alone is not enough to justify paying for analytics while we are still not making money.
I use Mixpanel to track interactions on my iPhone apps right now and it's really great. Highly recommended.