This is a free service from Google that gives a detailed analysis of your website, provided that you have implemented the Urchin javascript.

The main highlight is that a webmaster can optimize their AdWords advertisement and marketing campaigns through the use of GA's analysis of where the visitors came from, how long they stayed on the website and their geographical position.
Users can define and track conversions, or goals. Goals might include sales, lead generation, viewing a specific page, or downloading a particular file. By using this tool, marketers can determine which ads are performing, and which are not, as well as find unexpected sources of quality visitors.