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:
Neat. I've read that it's inspired in ImageOptim (OS X) and I love this app. It can strip up to 10% of the size of a 180k JPG photo and 60% of a tiny PNG. Will try.
There is one feature that I miss though, and it's some results about the compresion. Like "You saved 120KB (12%) of a total of 20 files (1024KB)".
