Google Analytics

Gogole Analytics will provide free hosted Web analytics service that businesses can use to track the effectiveness of online marketing campaigns

Some highlights of this service are:
- Free Service
- Learn how visitors interact with your website
- Identify the navigational bottlenecks in your site
- Finding how profitable your keywords are across search engines and campaigns
- Pinpoint where your best customers
- Easy-to-understand visually enhanced reports
- Available in different languages
- Integrated with Google AdWords

Google Analytics
http://www.google.com/analytics/

Features Detail
http://www.google.com/analytics/feature_fast.html