1. Double Click on the XL logo on the very top left corner to Close Excel.
  2. Adjust column widths by selecting multiple columns and double clicking on the separators (the thin line between the column letters).  This method also works for the rows.
  3. Auto-fill a series of cells with data or formulas using the double-click. Lets say you have a table where in one column you have some data and in the next you have written a formula in the first row. Simply, select the one cell then double click on the autofill box which can be found at the bottom right corner of the cell.  The mouse converts to a skinny black cross. The trick works for formulas, auto-fills (of numbers, dates, what not) as long as the adjacent column has data.
  4. Jump to last row/column in table with double-click. Just select any cell in the table and double click on the cell-border in the direction you want to go.
  5. Lock a particular feature and reuse them with double-click. You can lock any repeatable feature (like format painter, drawing connectors, shapes etc.) by just double clicking on the icon (in Excel 2007 this works for format painter, but for drawing shapes you need to right click and select lock drawing mode). This can save you a ton of time when you need to repeat same action several times.