What VoIP Software Does Conference Calls?

I need four things in a VoIP software client. 1) Must do conference calling (multiple phone calls). 2) Must be easy to use, or is able to work with other services. 3) Must either be cross-platform, or has clients for Windows, Linux, and Mac. 4) GPL or similar licensing preferred.

I know Skype can do conference calls, is relatively easy, and has clients for Windows, Mac, and Linux -- but it's not Free Software. Google Talk is free, but does not have a Mac or Linux client. Jabbin does not support conference calls.