Xymon monitors your hosts, your network services, and everything else.
Cisco switch port mapper
Simple software for port forwarding and routing w/o a hardware router
Xplico is a Network Forensic Analysis Tool (NFAT)
Pinger alert, STATICTICAL TABLES, send EMAILs or SMS by SMS GATEWAY
Open Source SIEM
Scalable, distributed monitoring system for high-performance computing
Embedded System based Temperature monitor
Tkined/Scotty is a graphical network monitoring package
...has moved to packetpeeper.org
Daemonlogger is a packet logger and soft tap developed by Martin Roes
Tool for identifying Network Service Dependencies
NDPMon - Neighbor Discovery Protocol Monitor