There is one keyboard shortcut that everyone should know about: the universal UNDO keyboard sequence: CTRL+Z (Command+Z on the Mac).

CTRL+Z ([CTRL]-key and Z-key pressed at the same time) undoes your previous edit. Multiple CTRL+Z’s will undo multiple edits and can even restore a document to an older revision, if it was changed during the same session. It works in almost every application…text editors, graphics editors, and even in games.

I thought it was a no-brainer that everyone knew about CTRL+Z until I had a recent encounter with a young relative who was struggling to recreate a project that she had accidentally erased before saving. A few “command+Z”s (on MacBook) and the document was back and I was a hero. I now realize that young people who grew up exclusively on GUI systems may not be aware of CTRL+Z and other keyboard shortcuts in the applications they use.

Along with CTRL+Z, there are 3 other basic universal keyboard shortcuts that everyone, young and old, should know. Like Ctrl+Z, these shortcuts work in almost every application, even when their associated action is not shown in the menu and is not given as an option on the screen.

Select All: Ctrl+A (Cmd+A on Mac)
Copy : Ctrl+C (Cmd+C on Mac)
Paste : Ctrl+V (Cmd+V on Mac)

Other keyboard shortcuts that are almost always universal and that I, myself, use frequently are:

Cut : Ctrl+X
Find : Ctrl+F
Repeat/Next: Ctrl+G
Save : Ctrl+S

Mac users must substitute “Cmd” for “Ctrl” when using the above keyboard shortcuts.

For further reading and additional keyboard shortcuts, here are some links:
Top 10 Keyboard Shortcuts everyone should know @computerhope.com
Table of Keyboard shortcuts @wikipedia.org
Universal Keyboard Shortcuts @Rutgers.edu
Firefox Keyboard Shortcuts @mozilla.org
Windows Keyboard – Explanation of Keys @seoconsultants.com
Keyboard Shortcuts – Mac Guides @macrumors.com
Apple Human Interface Guidelines: Keyboard Shortcuts @developer.apple.com