Non-Linux FOSS: Telephone
Whether you're trying to use Google Voice and a free SIP service to get a free telephone system or trying to connect to your company's internal phone system, if you're on a Mac, you need a SIP client. A few free options exist, but none are as simple as Alexei Kuznetsov's Telephone.
Telephone even integrates with OS X's Contacts program, so you can make a SIP call directly from there. It's free, open source, and it's even available in the Apple App Store. If you need to make a VoIP call on your Macintosh, be sure to check out Telephone: http://www.tlphn.com.