There is no substitute for an actual Pantone swatchbook if you are a) specifying color for a project and want to know accurately how it will print, or b) if you're putting together a color board for presentation to a client. There is no on-screen rendering that will show all PMS colors 100% accurately, or even some parts of the CMYK gamut -- the color gamuts don't completely overlap unless you're using one of the (extremely expensive) new Hewlett-Packard Dreamcolor monitors.
You will also see different renderings of PMS spot colors depending on which application you view them in. Photoshop uses the Lab values specified by Pantone, where Illustrator and InDesign, by default, render spot colors using Pantone's CMYK conversion values. Many a designer has been dismayed when placing a carefully crafted Photoshop image with spot colors, only to find the colors look different in InDesign than they did in Photoshop.
To make the displays match, you can tell Illustrator or InDesign to always display spots using Lab values, to make them consistent with Photoshop, or (preferably) you can turn on "Overprint Preview" in the View menu of either program.
I can't improve on Mordy Golding's excellent article on this subject here, which also covers how you can get the right values in the composite proofs you print locally.