SmokePing
RRDtool
Big Sister
Nagios
Pandora FMS
Performance Co-Pilot
Network Weathermap
Icinga
RANCID monitors a router's (or more generally a device's) configuration, including software and hardware (cards, serial numbers, etc) and uses CVS (Concurrent Version System) or Subversion to maintain history of changes.
GitHub - ssinyagin/gerty A universal framework for device management automation. Eventually a replacement for RANCID… and much more. (last commit Jan 11, 2017).
Xymon
GitHub - Netdata

NfSen is a graphical web based front end for the nfdump netflow tools. (License: BSD)
Flow-tools is a software package for collecting and processing NetFlow data from Cisco and Juniper routers.

AreWeDown.com (dead links) service that runs on XP workstations, Windows 2000 workstations/servers, and Windows 2003 servers. The service logs database requests to a MySQL server every 30 seconds.
NetAdminTools.com (dead links) represents the world of network administration in a pleasant way, particularly for those who find the idea of monitoring systems with red seven-segment displays, LEDs, and green/amber text written in courier. NetAdminTools.com is simply a console that accepts commands from localhost on port e (27183).

23rd Chaos Communication Congress - A 10GE monitoring system
24th Chaos Communication Congress - 10GE monitoring live!

InterMapper

Paessler voor:

• PRTG Traffic Grapher (freeware editie beschikbaar, beperkt in functioneren)
• IPCheck Server Monitor (freeware editie beschikbaar, beperkt in functioneren)
• SNMP Helper (freeware editie zit in freeware editie van PRTG Traffic Grapher en IPCheck Server Monitor)
• Webserver Stress Tool (freeware editie beschikbaar waarin user simulation beperkt is tot 10 gebruikers)

networkTAPS.com
Anue Systems
NetOptics
Endace
Force10 Networks - P-Series
Napatech
FT-system wiretap solution - all-speed Ethernet and optical fiber tap units