Google Analytics
Ars Technica is running an article on the new Google Analytics service that provides traffic metrics and other website analysis features, especially useful for marketing purposes. It also claims to provide easy integration with Google AdWords to help find out the usefulness of the campaigns.
Web sites that don't use AdWords can add a small javascript snippet to the pages they wish to track.
Google is using the Urchin web analytics technology, a company it had earlier acquired.
Also discussed on Slashdot here.
More on this once I've integrated it with this blog.
Update: Brief coverage on TechCrunch.