Vonage offers a program (Click-2-Call) that will dial numbers directly to Vonage's server via the desktop; however, they don't offer a Mac version of this program. I came across this Address Book script that will allow you to dial directly from Address Book.
[robg adds: I don't have Vonage, so I can't test this one; the scripts (there are also dialers for Skype and Xten Softphones) are freely downloadable and cost only $1.50 in shareware fees if you find them useful.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050706215557904