Do basic math/stats operations on a series of cell values in a Google Docs table and insert the result with the click of a button. 

=>Operations include addition, multiplication, average, count, max, min and standard deviation
=>Works with currency symbols of several countries  
=>A bonus regular calculator 
=>Works for both US (period) and non US (comma) separators for the decimal system 

Stay within Google Docs and do all your simple math, no need to get a separate calculator or use a spreadsheet.