Nagios - Network Monitoring

Nagios is great Network Monitoring tool. At the most basic level it pings a few devices and mails you to let you know there's a problem. But it can do so much more - check pretty much anything and then take action on the result. Many supplied functions allow you to check DNS, SMTP mail, Response time etc. etc. I've created scripts of just a few lines to alert me if router CPU exceeds thresholds (that can be set on a per device basis). Runs on Linux and its free!. Highly recommended.

 MRTG - Network Graphing

MRTG is the original tool for graphing traffic stats and more on your network. Simple to setup and use but it has its limitations. Again its free (Thank you Tobias Oetiker) but runs on Linux or Windows.

 Cacti - Network Graphing

Cacti uses RRD (by MRTG Author Tobias Oetiker) for producing graphs of network traffic and other stats, but adds a great front end configuration and viewing packages aswell as doing the backend Data Collection. Its great strength is the use of templates to allow replication of the same graph styles across many data sources - aswell as just sheer ease of use! Again its free (Thank you Ian Berry) and runs on Linux or Windows.

 Cacti - Custom Templates

Custom Templates and Queries I've created for use with Cacti...

 IPCop - Network Security

A cut-down Linux distro that turns an old PC into a dedicated Firewall appliance. Unbelievably easy to set up and configure and... its free

 IPCop - Configuring GRE Tunnels

Recently I did a fair amount of work using IPCop and its easy to configure IPSec VPN to connect some sites across the Internet. Unfortunately I needed more IP routing flexibility than that configuration allowed so I added some GRE tunnels on top. I now have both secure and flexible inter-site tunnels. I though others might find it interesting/useful so have put the details down here...