The Custom Formats. Colors.app can both input and output color values in multiple formats (Hex, RGBA, HSL... you can even create your own), making much easier working with colors between Photoshop and CSS.

It's basically a nice interface for the system color picker.

I discovered this app not long ago in one of the comments in the following article; which covers other little gems too, definitely worth reading if you're into front-end design:

http://www.macstories.net/reviews/spot-color-use-mac-os-x-co...

(sorry about the delay, it's hard to keep track of conversations here! :)