I've been using Skype on my Mac lately to make voice calls to other Skype users. That works so well that I just purchased SkypeOut, which allows me to call directly from Skype to any US phone number for the flat rate of USD $30/yr. Seems to work pretty well.
Skype comes with a plugin that allows you to dial phone numbers from your Mac address book, but it is broken. For example, it will only dial numbers that don't have spaces or dashes in them.
I found some info on the web and a replacement script to fix that, but the replacement script ate some digits of the phone number when I tried to use it. So, I fixed it. Here it is as a compiled script and here is the text version. Also my version is for use in the US, you need to replace '+1' with your country code if you want to try this somewhere else.
To use this, drop the script in your user Library/Address Book Plug-Ins folder and restart the Address Book. I suggest before you do that you delete the script that comes with Skype, which is located in the global /Library/Address Book Plug-Ins. Otherwise you will see two scripts listed in the Address book. This won't break anything but it is confusing.
To use this script, simply ctrl-click (or right-click) on any phone number in the address book and choose Dial this number in Skype.