These are for Excel on Windows. On a Mac, most combinations swap Ctrl for Cmd, though a handful (like F4) stay the same either way.

General
ShortcutWhat it does
Ctrl + N / O / S / PNew / Open / Save / Print
Ctrl + Z / YUndo / Redo
Ctrl + `Toggle showing formulas instead of results
Navigation & selection
ShortcutWhat it does
Ctrl + Arrow keyJump to the edge of the current data region
Ctrl + Home / EndJump to A1 / the last used cell
Ctrl + Page Up / DownSwitch between sheet tabs
Ctrl + SpaceSelect the entire column
Shift + SpaceSelect the entire row
Ctrl + Shift + 8Select the current region (the whole table at once)
Formulas
ShortcutWhat it does
F4Toggle a cell reference between relative and absolute (adds/removes $)
Alt + =AutoSum the selected range
Ctrl + EFlash Fill — finish a pattern automatically
F2Edit the active cell directly
Data entry & editing
ShortcutWhat it does
Ctrl + DFill down from the cell above
Ctrl + RFill right from the cell to the left
Ctrl + ;Insert today’s date as a fixed value
Ctrl + Shift + ;Insert the current time as a fixed value
Alt + EnterStart a new line inside the same cell
Formatting
ShortcutWhat it does
Ctrl + 1Open Format Cells
Ctrl + B / I / UBold / Italic / Underline
Ctrl + Shift + $Apply currency format
Ctrl + Shift + %Apply percentage format