http://awstats.sourceforge.net/ 
Free
AWStats is a free feature rich log analyzer tool which generates statistical reports from your ftp log files, server logs or mail log files. This tool is a real time log analyzer which supports has the capability of processing log files quickly from servers like apache, webstar, IIS and many more.
- Total number of visits and unique visitors
- Time spent by visitors and their last visits
- Visits from different country and hosts
- Operating system used
- Internet Browsers and their support ratio
- Robot visits detail
- Most views pages
- Entry and exit
- Visits from authenticated users
- List of hosts and unresolved IP addresses
- Visits of robots
- Search engine keywords
- Http Errors