Task-Manager (a.k.a. System Monitor)
🔍 + Esc (allows you to terminate processes or apps)
Screenshot
Strg + F5
or on Chromebooks Strg + [ ]]] (window overview key)
Screenshot of part to be selected
Strg + Shift + F5
or Strg + Shift + [ ]]]
then a part of the screen can be selected with the mouse
Screen Lock
Shift + CapsLock + L
or Shift + 🔍 + L
Keyboard overlay
Strg + Alt + ?
Then press modificator keys (Shift, Alt, Ctrl, AltGr) to see keyboard shortcut available with that modificators.
Unicode Character Input
Strg + Shift + U nnnn ENTER (where nnnn is a hexadezimal chracater code, e.g. 2022 for a • symbol)
This is explained in detail in the article Unicode Character Input.