Google Analytics is far and away the most common statistics package for webmasters to use. Thats particularly true for ecommerce webmasters who often appreciate the fact that it integrates with AdWords.
There’s one problem: using GA in a mambot or placing the code inside templates doesn’t work well if you’re using an SSL Certificate.
Sometimes the tracking is lost and sometimes it leads IE to produce errors (usually pop-ups saying "Some parts of this page contain unsecure information. Do you wish to continue?")