Here I will give you some regards that I found useful

Learning about networking

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.

Yeah... I know... you always will learn you should not use tools at the networking courses but each brain needs a break sometimes.
So it's fine to have a little helper if you're planning a new network.
"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) -> this baby is hot and protecting me each day (free bl. lists available)!
Backup can be done very easy and it will save everything - you can protect the backup file of course with a password. I LoVe 17!
Post sc3ipTuM: The products of pfSense are also fine ^^ - take a look at it .

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 :) - RTM => you have to do a manual installation.

The Morpheus tutorials (german) - the best video tutorials I know about programming and hacking. Thx a lot 2 "Morpheus"!
 The Morpheus