Google Analytics

Google Analytics is a powerful, free tool that provides insight into website traffic by allowing advertisers, publishers, marketers, content developers and/or site owners informational features via customizable reports. Applicable not only for Google AdWords, but to assist in focusing marketing resources on campaigns and initiatives that deliver ROI.

From the website:

Google Analytics helps you find out what keywords attract your most desirable prospects, what advertising copy pulled the most responses, and what landing pages and content make the most money for you.

... and ...

Google Analytics is the enterprise-class web analytics solution that gives you rich insights into your website traffic and marketing effectiveness. Powerful, flexible and easy-to-use features now let you see and analyze your traffic data in an entirely new way. With Google Analytics, you're more prepared to write better-targeted ads, strengthen your marketing initiatives and create higher converting websites.

Features:
- Advanced Segmentation
- AdWords Integration
- Benchmarking
- Custom Reports
- Custom Dashboards
- Ecommerce Tracking
- Email Reports
- Fast Implementation
- Funnel Visualization
- GeoTargeting
- Internal Site Search
- Keyword and Campaign Comparison
- Motion Charts
- Site Overlay
- Trend and Date Slider

Highly recommended for the serious webmaster!

Visit Google Analytics