A lot of *Nix Documentation and Know-hows, command snippets, code…
System Administration (Cheatsheet)
* Packages
* Database Administration
+ :::: MySQL/MariaDB
+ :::: Postgres SQL
* Virtual Environments / Containers
+ :::: Linux chroot
+ :::: QEMU/KVM
+ :::: Docker
+ :::: Podman
* Web Servers
+ :::: Applications
+ :::: Servers
* Programming / Scripting
+ :::: Bash
+ :::: Git
+ :::: Python
+ :::: CURL
+ :::: HTML/CSS
+ :::: Javascript
* Desktop Management
+ :::: Xorg
+ :::: i3 Window Manager
+ :::: Printers/Printing
+ :::: PulseAudio
* Startup / Persistance
* Disk Management
+ :::: Privacy
* Memory Management
* Networking
+ :::: Firewall
+ :::: Relays/Proxies
+ :::: Misc
* Data Processing
+ :::: SSL/TLS
+ :::: Images
+ :::: Documents
+ :::: Backups
+ :::: Files
* (Offensive) Security
+ :::: Packet capturing / analizing
+ :::: Hacking & Cracking
Applications
* GNU Linux
Tutorials
* Android
* Linux
+ :::: Security
+ :::: Networking
+ :::: Misc
Snippets
* Programming
+ :::: PHP
+ :::: Python
+ :::: Protocols
Others