Recs.
Updated
Google Analytics (GA) is a free service that does a great job at high level analytics such as traffic sources, visitor counts and statistics on your content. However the event-based tracking is not as sophisticated as Mixpanel or Kissmetrics, so GA is usually used in conjunction with one of those services as well.
Specs
Pros
Pro Import data
Google Analytics allows users to update user information from other sources.
Pro Awesome API
The API itself, tools, documentation, and abilities of the API are very powerful.
Pro Provides free training
Google provides free services that train users in each aspect of Google Analytics, especially when used in conjunction with AdWords.
Pro Allows custom dashboards
Depending on the most important information to the user, the dashboard can be customized in order to put emphasis on certain metrics. This feature allows Google Analytics to satisfy users from a broad range of industries.
Pro Free for most use cases
Google Analytics is free unless you need their premium service, which is $150,000 per year. However, it's only necessary for massively large enterprise size data sets.
Cons
Con Heavy users get sampled reporting
If you send a lot of data Google starts sampling the data it uses for your reports.