Skala Color

Skala Color is a compact and feature-rich OS X color picker that works with a huge variety of formats, covering everything you're likely to need for web, iOS, Android, and OS X development -- Hex, CSS RGBA, CSS HSLA, UIColor, NSColor and more.

It also automatically recognises colors copied to the clipboard, presenting them as a swatch that can be applied with a single click. The hue and opacity sliders let you quickly target rough values, then fine tune with the precision.
What's new in this version:
Added format restoring -- format choice is now remember across launches. Fixed a crash in some apps (Final Cut Pro, iMovie etc). Fixed other small bugs.