Most people know cut, copy, paste, undo.
A few I've learned recently, is clicking with the mouse wheel in Firefox and IE7 will open a link in a new tab. And if you mouse wheel click on a tab, it will close it.
F2 will place the cursor into the cell in Excel. Ctrl-(arrow key) will take you to the blank space in a row or collumn of data. (Or if you're on a blank space, it'll take you to the next cell with data.
So many of us have these little tricks we've learned over time, but don't share them. What shortcuts do you know?
EDIT: A couple others that came to mind:
I'm sure most know the beauty that is ALT-TAB.
In a browser you can change the text size by CTRL-(rotate mouse wheel up or down).