If you're lucky enough to have a Grand Central account, here's a quick PHP class that will login to your account and dial a phone number. This is probably one of the more random bits of code I've ever written, but I think it's useful.
<?PHP
$gc = new GrandCentral('gc_username', 'gc_password');
$gc->call($your_number, $their_number);
And that's it. Grand Central will call $your_number and connect you to $their_number.
So, you can dial phone numbers from PHP. Now what?
Well, for starters, here's an AppleScript Address Book plugin that will let you dial a contact from your Mac. Or, if you're like me and have lots of conference calls each week, you can attach a script to your iCal events to dial you into the conference number a minute or two prior to the call start time.
As usual, the code is MIT Licensed and available in GitHub.
Contact me if you have any questions.