Skala Color 1.0 (version history). OS X 10.8+ required.
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 4× the precision.
Skala Color is a standard OS X color picker, so it works with most Mac apps. Skala Color is free, to help spread the word about Skala, our upcoming design tool.