Here are 22 keyboard shortcuts varying in usefulness.

Cell Formatting

Shortcut What it does: Usefulness
Ctrl+Shift+&
Applies the outline border to the selected cells.
Ctrl+Shift_
Removes the outline border from the selected cells.
Ctrl+1
Displays the Format Cells dialog box.

Adding and Editing a Comment

Shortcut What it does: Usefulness
Shift+F2
Adds or edits a cell comment.

Date

Shortcut What it does: Usefulness
Ctrl+Shift+#
Applies the Date format with the day, month, and year.
Ctrl+;
Enters the current date.

File Commands

Shortcut What it does: Usefulness
F12
Displays the Save As dialog box.

Number Formatting

Shortcut What it does: Usefulness
Ctrl+Shift+~
Applies the General number format.
Ctrl+Shift+$
Applies the Currency format with two decimal places (negative numbers in parentheses).
Ctrl+Shift+%
Applies the Percentage format with no decimal places.
Ctrl+Shift+^
Applies the Scientific number format with two decimal places.
Ctrl+Shift+!
Applies the Number format with two decimal places, thousands separator, and minus sign (-) for negative values.

Text Formatting

Shortcut What it does: Usefulness
Ctrl+2
Applies or removes bold formatting.
Ctrl+3
Applies or removes italic formatting.
Ctrl+4
Applies or removes underlining.
Ctrl+I
Applies or removes italic formatting.
Ctrl+U
Applies or removes underlining.

Time

Shortcut What it does: Usefulness
Ctrl+Shift+@
Applies the Time format with the hour and minute, and AM or PM.
Ctrl+Shift+:
Enters the current time.

Navigating the Workbook

Shortcut What it does: Usefulness
Ctrl+PgDn
Switches between worksheet tabs, from left-to-right.
Ctrl+PgUp
Switches between worksheet tabs, from right-to-left.
F1
Displays the Excel Help task pane.

Summary

Keyboard shortcuts allow you to perform nearly every command in Excel without moving your hand back and forth to the mouse from the keyboard. Learning and using a few shortcuts can make your work much more efficient.