CalcService is a great add-on which allows you to calculate the result of a mathematical expression right within a Cocoa app. I've used this for some time in my personal finance program: to add up a couple of figures, highlight them, press the keystroke equivalent, and boom -- the result is there. No need to open a separate calculator, etc.
Just recently it occurred to me that you could put this to use in many other ways. How many people resize images on a regular basis? How many people have to quickly pull out a piece of scratch paper to figure out what half (or a third, etc.) of 537 pixels is? If you're working in a Cocoa image editing/manipulation application (such as Keynote or OmniGraffle), and your image-resizing controls are text fields, just take the number of pixels already listed, type *.5, highlight and invoke the service, and boom -- your image is resized by half!
Simple, but saves a lot of time...

