Google Analytics tells you everything you want to know about how your visitors found you and how they interact with your site. You'll be able to focus your marketing resources on campaigns and initiatives that deliver ROI, and improve your site to convert more visitors.

I just sign up, and use it to this JUNAIDIX.NET website. Installation is simple, just copy and paste some javascript code to your webpage.

First time use, so i had to wait min 12hours to see my first chart of my visitor online.

More info: Google Analytics

Other available free website traffic analysis which i know:
ExtremeTracking.com
Webstat4u.com
OneStat.com