Comparison of free Web Analytics tools

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There are a bunch of free Web Analytics tools -

• Google Analytics 
• StatCounter 
• Piwik 
• Woopra 
• Clicky
• SiteMeter

Each of them has something unique. The Clicky website compares all the above across some 40 odd parameters. (updated broken link on 1/Oct/2013)

Except for Google Analytics & Piwik, the rest have limitations on features with their free editions. They restrict the data retention size & period.

Piwik is a downloadable, open source (GPL licensed) real time web analytics software program that uses PHP & MySQL. It needs to be installed on your server, so you own your data.

The number of hits captured by each of the tools for the same site could vary. This could be because some tools like Google Analytics use JavaScript & a user may disable it in their browser. Some of these may not count bots and some register hits based on user's visit length.

Also see:
HOW TO generate a Heat Map with Google Chart API