Mac: The "Sum" function in spreadsheets is useful, but we don't all make our lists in Excel. Fortunately, Coder Brett Terpstra has built an OS X System Service that allows you to total up any set of numbers in any document.
Once the system service is installed, you can start selecting and totalling up text. Just select a block of numbers, right-click it and choose "Total Numbers". You'll then get an output with the total. If you're the type to make lists in plain text or Markdown, this is a great way to sum up a total without dropping it all into a spreadsheet or pulling out a calculator.
A Service for getting sums from selections [Brett Terpstra]