Google has released a new application for Mac users called "Vocito" that puts GrandCentral calling right on your desktop. It integrates with OS X's address book, Automater app, and third-party applications like Blacktree's QuickSilver to let you start a GrandCentral-powered call no matter what you're doing on your machine.
Similar to JaJah and Jaxtr, Vocito's system for setting up calls involves you first picking who you want to call, then choosing which one of your GrandCentral-connected phones you want it to be connected from. You then hit dial and GrandCentral does the rest. It's basically the same exact thing you've been able to do with GrandCentral's Web interface for years, but now you can have a deeper level of integration across your entire system.
This deeper integration centers around a slick and simple Mac taskbar drop-down application that's directly integrated with the Address Book app. It lets you start a call almost as fast as doing a Spotlight search. This is made a little easier if you're a Quicksilver user, since you can search for contacts and call them via Vocito with the included plug-in. In most cases, this worked for me with about six keyboard strokes, which I found faster than picking up my phone to dial a contact.
If you don't have time for six keystrokes, the application lets you save a customized phone call preset as an AppleScript command. Clicking it begins the call immediately. You can leave these laying around your desktop, or put them into a folder to keep in a stack on your dock for handy "speed dialing" later on.
Vocito is free to use and will run on both PPC and Intel Macs. You will, however, need a GrandCentral account, which Google continues to keep in a highly limited beta--that is, unless you're homeless.