google analytics
This is pretty insane. The integration is cool and the overlay technology is very cool.
http://www.google.com/analytics/feature_fast.html
I'll be switching over shortly.
Update: I won't be moving over too shortly because the site seems to be down. Probably being crushed under the load. I'm getting a "Currently Undergoing Maintenance" page. I should probably wait anyway until some of the reviews come in.