Mac OS X only: Free, open-source application Vocito puts Google's one-phone-number-to-rule-them-all application GrandCentral on your desktop, integrating directly with your Address Book contacts. The most obvious way to place a call once you launch Vocito and enter your GrandCentral login is to click the new menu bar item, search for the contact you want to call, and place your call—but it's by no means the only way. Beyond Address Book integration, Vocito also works with Quicksilver (awesome), Automator, and AppleScript. We haven't seen a lot of movement from GrandCentral since Google acquired it last year, so it's exciting to see even a little action from them. Vocito is a free download, Mac OS X only.
Vocito [Google Code via CNET]
