Skype is now out with a free Linux version of its VoIP technology which allows PC and now Linux users to talk freely over good Internet connection lines. For the many others out there seeking alternative solutions that would enable them to talk or videoconference with colleagues, teammates and friends across multiple operating systems here are a few recommendations:

While these are not open-source, free solutions, most of them offer a free try-out and a very cost-effective fee for their use.
Please consider:

1) Marratech
This is a fully secure collaboration space that supports both VoIP and videoconferencing. You may want to try immediately by using one of the several available test rooms. Kolabora.com Direct access from here or here (Win, Mac, Linux)

2) Flash Communication Server -FCS offers full web-based VoIP and videoconferencing across mulitiple platforms. To see how easy it is to use go ahead and test it from any major OS on anyone of these online services:
2a. Uvault Demo Rooms

2b. Megameeting

2c. E-boardroom

3) iVisit -
This is a very cost-effective audio/video application which integrates the ability to create private spaces. (Win & Mac only)

4) Convoq ASAP -
This is a web conferencing and live presentation solution. While the presenter needs to be on a PC, attendees can join from any major OS. Convoq integrates VoIP and videoconferencing using FCS.