No matter how well organized or spec'ed out a PSD is, there's always that point where you have to manually bring colors from design to code. Here are a few tools that have helped me get a seamless 360° workflow for dealing with colors in OS X.
Alfred 2 Colors Workflow
Amazing workflow by Tyler Eich that allows you to preview, modify and convert colors in a bunch of different color spaces and formats (rgb, hsl, hex, ...). Great if you want to convert those hex values to RGB for CSS.
Hex Color Picker
Adds a hex field to the native color picker across the OS. Perfect for pasting hex values in Xcode Interface Builder, for example, without the need to convert to RGB and manually paste all three channels.
Apple Script to Launch Color Picker from Anywhere
Create the following one-line Apple Script and export it as an Application to launch the color picker from anywhere (e.g. Alfred or Spotlight). I didn't like the generic AppleScript icon, so I added a custom HSL wheel from Google images by pasting it into the the info panel of the exported app bundle.
choose colorXcode OMColorSense Plugin
Install this using Alcatraz. Great plugin that previews colors inline and allows you to easily change values using a picker and modifying your code on the fly. I recommend watching the demo video below. It's super seamless and has saved me tons of manual conversion and copy/paste.