The universal set

• Ctrl+C / Ctrl+V / Ctrl+X, copy, paste, cut
• Ctrl+Z undo, Ctrl+Y or Ctrl+Shift+Z redo
• Ctrl+S save. Press it constantly and by reflex
• Ctrl+F find on this page or in this document
• Ctrl+A select all

The ones fewer people know

• Ctrl+Shift+V, paste WITHOUT formatting. Ends the copied-from-Word mess forever
• Ctrl+Shift+T, reopen the browser tab you just closed
• Alt+Tab. Switch windows; hold Alt and keep tapping Tab
• Win+Left / Win+Right, snap a window to half the screen
• Ctrl+Backspace, delete a whole word
• F2, rename the selected file

In a text editor

• Home / End, start or end of the line
• Ctrl+Left / Ctrl+Right, jump a word at a time
• Shift plus any of those, select while moving

The arithmetic

Reaching for the mouse costs a second or two. Twenty times an hour, every working day. That is days per year. This is the cheapest speed increase available to anyone who uses a computer.