Alt and Return: Start a new line in the current cell instead of moving to the next one.
F11: Generate a chart from the currently selected data.
Shift/Ctrl and Space: Select the current row/column.
Alt and =: Total the selected cells.
Ctrl and Shift and $: Format selected cells as currency with two decimal places.
Ctrl and ;: Insert current date.
Ctrl and Shift and ': Paste value from the cell above.
Shift and F11: Insert a new worksheet.
Ctrl and PageDown/PageUp: Move between worksheets.
Ctrl and D/R: Fill cell with the contents of the cell either above (D) or to the left (R).