With version 2.1 of AsteriskC2D for OSX you can try it out for 10 days, after which time you need to pay for a license key.

$30 for an unlimited license.

Dial from almost any Mac Application

Using the services Menu you can select any a number from almost any application and request your PBX to dial it for you. Don't forget to include the + in the selection for international dialing support!

If that's not enough for you, you can write your own bits to call the dialer application with the phone number and it will do the rest. Maybe a plugin for Firefox, or a widget, the possibilities are endless. If you do write stuff, please let me know so they can be added to the Plugins page.

Support for Asterisk and derivatives built in as well as thirdlane, Vonage, Fonality.  Other PBX types can be added via a super simple plugin system.