There are a few tools I use day-to-day that I couldn't be without, one of them is a colour picker to use on the desktop. Traditionally I used the one that ships with Mac OS X called Digital Color Meter, however at around the time Lion was released there was an update which dropped functionality I was heavily reliant on. The Mac App Store was still in its infancy, but after a few searches I'd found the replacement I was looking for — Classic Color Meter.
It promised to replicate the original functionality of the Digital Color Meter, allowing me to essentially replace the application with this one and carry on as before. Since then it's grown and added some extra functionality which should make it useful to designers and developers across the board.