Monitorix is a free, open source, lightweight system monitoring tool
monitorix, which is a Perl daemon that is started automatically like any other system service, and a CGI script called
monitorix.cgi. Since 3.0 version Monitorix includes its own HTTP server built in, so you aren't forced to install a third-party web server to use it. All of its development was initially created for monitoring Red Hat, Fedora and CentOS Linux systems, so this project was made keeping in mind these type of distributions. Today it runs on different GNU/Linux distributions and even in other UNIX systems like FreeBSD, OpenBSD and NetBSD. It is currently in active development adding new features, new graphs and correcting bugs in the attempt to offer a great tool for daily systems administration. Monitorix is an open source project and, just like any other open source project, anyone can contribute with his own time and knowledge.
Find out more about Monitorix here.