Any color picker will let you choose a color from anywhere on your screen, but Developer Color Picker turns that into usable code for your Xcode and web development projects. Developer Color Picker generates code suitable for NSColor, UIColor, CGColorRef, CSS and HTML declarations.
Depending on what you want, you can copy just the value for the color itself, or an entire declaration including the color. Imagine the time you save not having to type UIColor *aColor = [UIColor colorWithRed:0.282 green:0.569 blue:0.894 alpha:1.000]; again!
Developer Color Picker is available on Panic's website.