Is there an OS X tool for finding coordinates, pixel color and measuring distances on the screen?
You can show screen coordinates if you start taking a screenshot by pressing ⇧⌘4:
You can also use it to measure the width and height of areas:
To abuse bundled applications even further, it's easy to get the diagonal in AppleScript Editor:
(114 ^ 2 + 89 ^ 2) ^ 0.5
You can see the color of pixels with DigitalColor Meter:
Coordinates | Digital Color Meter -> View -> Show Mouse Location