The built in Apple color picker has a Pantone color picker built in! Go to the 150-Line (Pantone) from the swatches pallet tab. At least on Tiger... So you can use it in ANY application that uses the Apple color picker... TextEdit, Pages, MS Word, etc. It's a bit limited, but these applications don't really print spot colors anyway, with the exception of Pages (which I haven't tried).

And any application that can actually print a spot color, i.e., QuarkXPress, InDesign, Illustrator, etc., already has spot color library built in. Even Photoshop has it.

Other applications, such as MS Word, can't print spot colors and therefore something like this is only good for getting the approximate RGB simulation of the spot color. So it could be useful for getting the same look as a Pantone color, without actually using a spot color. But you could also just mix one by eye, since you are not really printing with a spot color. But as I pointed out, OS X already has this feature built in.

Many people are unclear as to what a spot color actually is, and when to use them or not use them... It's a premixed ink that is not made up from CMYK, and prints as its own plate, so an orange spot color is orange ink, and not yellow and magenta mixed ... and a four color job (CMYK) would not have any spot colors, even if you are using the same color as a spot color (it would be separated) but a 2 color job (black and PMS 185 red for instance) would. Got it? Good! :)