I’ve been using Excel since version 1.0 so I’m pretty proficient, but there’s so much functionality that’s been added over the years that it’s pretty hard to utilize it all. For example, here are two helpful tricks that I’ve never heard of before:

1. Pressing Alt and the equals sign at the end or bottom of a row or column of numbers will insert a sum formula for you.
2. Holding down the Shift key and then clicking on the File menu will reveal a Close All item to quickly close worksheets.

For some more useful tips, have a gander at Getting the Most Out of MS Excel at Web Worker Daily.