Ctrl+Z (or Command+Z on the Mac) is the best way to undo the last thing you did on many programs - the last thing you typed in Outlook, Word, and gmail, the last cell you edited in Excel, the photo you accidentally deleted from your document, and more. And if you accidentally press Ctrl+Z and want to redo the thing that just got undone, you can always hit Ctrl+Y (Command+Y on the Mac).