Change sign  
 
This useful tool allows you to change the sign of a range or cell containing formulas or values.
If the cell contains a formula, a parenthesis will be placed before changing the sign, while the formula is kept unchanged.
Note the following example:
The following range is selected:
 
 
First click:
The values of all cells now have negative values
 
 
Second click:
The values of all cells are now positive again