AWStats is an audience tracking tool based on analyses of web logs. It was written a number of years ago by Laurent Destailleur.
AWStats provides a wide range of statistics, graphs and reports generated from its analysis of web logs (as well as FTP, streaming and email). It can natively read the log files of most web servers such as Apache, WebStar, IIS, etc. Its functionalities include measurements of the number of visits, unique visitors, pages, hits, transfers, by domain/country, host, time, browser, OS, and so on. One of the strengths of AWStats lies in the possibility of dynamically generating tables without losing out on performance, thanks in particular to an efficient caching policy. It is very popular among systems and network administrators.
The AWStats open source software is available under the GPL.
Written in Perl, AWStats can be installed and run on most systems. It is a highly mature supervision tool with packages on all Linux distributions.