I met this excellent Debian based distribution called Kali Linux. I found some of its features very helpful to test the security of my projects. And I especially love it because I can access it for 100% free. Since I just began with this OS, I'm aware only a few features that I checked randomly.

What are the tools that you use from this distribution?
Do you have any good tutorial for that tool?

I'd prefer to pick and learn the most useful tools, not all if not necessary.