Cacti: The Complete RRDTool-based Graphing Solution --http://www.cacti.net/
Cacti is a complete frontend to RRDTool, it stores all of the necessary information to create graphs and populate them with data in a MySQL database. The frontend is completely PHP driven. Along with being able to maintain Graphs, Data Sources, and Round Robin Archives in a database, cacti handles the data gathering. There is also SNMP support for those used to creating traffic graphs with MRTG.
Icinga is a scalable and extensible monitoring system which checks the availability of your resources, notifies users of outages and provides extensive BI data.
Munin the monitoring tool surveys all your computers and remembers what it saw. It presents all the information in graphs through a web interface. Its emphasis is on plug and play capabilities. After completing a installation a high number of monitoring plugins will be playing with no more effort.
Nagios is an enterprise-class monitoring solutions for hosts, services, and networks released under an Open Source license.
SecTools.Org Top Network Security Tools --http://sectools.org/
the Nmap Project has been cataloguing the network security community's favorite tools. In 2011 this site became much more dynamic, offering ratings, reviews, searching, sorting, and a new tool suggestion form. This site allows open source and commercial tools on any platform, except those tools that we maintain (such as the Nmap Security Scanner, Ncat network connector, and Nping packet manipulator).