The Webalizer is very good at mining your Apache logs for stats -- who visits, from what IP addresses, a breakdown of the kinds of domains those IPs reverse-resolve to (.com, .net, .co.uk, .ru, etc.), what their browsers claim to be (which is usually, though not always, what they actually are), what operating system the user is using (again, provided the browser is reporting it honestly), etc., etc. It's completely non-intrusive; nothing special on your web pages, no additional requests to external servers on every page load, etc.
Those would be two of the big ones, each with its different approach. The Webalizer being entirely local (and therefore private) and non-intrusive may be appealing; the scope and analysis features of GA may be appealing.