Chroma is an OS X Dashboard utility for converting colours between HSV, RGB, and HEX formats. Favorites can be saved to the swatch library, and traded between computers using clipboard import/export options.
Colour data can be added a number of ways, from pasting from the clipboard (such as when copying HEX values out of Photoshop), entering numbers manually, or using the arrow keys to increase or decrease a value (hint: hold down the shift key for larger multiples!).