Ask HN: OSX status bar calculator

What if you could hit a simple keyboard shortcut and a text field dopped down from the status bar where you could type in arithmetic, press return and see the result. When you are done, another shortcut closes the window and you return to Sublime Text/Xcode/TextMate...

I've tried a whole bunch of calculator apps and find the workflow of switching windows to be a pain. It's crossed my mind a couple times, so I thought I'd see if you guys would find this useful, and if so what features would you want. (Of course if I've somehow missed something like this that already exists please let me know.)