Here I will give you some regards that I found useful

Learning about networking - here you will find all you need

Some people call it the swiss army knife internet tool ^^

There is anything faulty with the page or ip you checked at your firewall logs... time to get more informations...
This is also a good monitoring tool (monitoring to one ip is free). Check out the tools of the famous MX Toolbox ;D

Yeah... I know... real men and women using there brain but dude - each brain needs a break sometimes.
So it's fine to have a little helper if you're planning a new network ^^ ACK KISS!
"Use is it when there is no storm to be shielded when it storms..."

Looking for a god damn great firewall based on pf and BSD? - Try pfSense!
You can also install an IDS/IPS (Snort) -> damn this baby is hot and protect me each day (free lists available)!
Backup can be done very easy and it will save everything - you can protect the backup file of course with a password. LoVe 17!
Post Sc3ipTuM: The products of pfSense are also fine ^^.

Geeks for Geeks - "A computer science portal for geeks" =)

Open-Source-Network monitoring system - one of the best

Another open source monitoring system - lightweight -> good for RPis :)

The Morpheus tutorials (german) - the best video tutorials I know about programming and hacking
 The Morpheus