Everyone uses Excel a little bit differently, so a trick that’s a big time saver for one, may not do much for someone else. But here are three (in no particular order) that I find myself using frequently. If your favorite didn’t make the list, I’d love to hear it.

Ctrl+Home or Ctrl+End moves you to the beginning or the end of your spreadsheet respectively much faster than you can using the mouse. Yes, those keeps between the letters and the number pad do have a purpose! Bonus tip: combine either one with the shift key and you will select everything from the current cell to the beginning or end of the spreadsheet.

Ctrl+; (semicolon) to enter the current date. If that seems too obscure to remember, it’s corollary will help. Ctrl+: (colon,like you would use when typing the time) is used to enter the current time. So Ctrl+: and Ctrl+; are related just like time and date are related.
Alt+= will sum a column or row. If you are at the bottom of a column or end of a row, Alt+= will automatically insert the sum function for you. One warning, if there is an empty cell in the column or row, the formula will only sum the cells below (or to the right of) the empty cell.

For those of you who are completely mouse-centric, you don’t actually type the + in any of the shortcuts. Just hold down the Ctrl or Alt key while you type the appropriate shortcut key.