Cross-Platform, Simple Voice Chat Software?

nordicfrost asks: "My wonderful girlfriend and I have a dilemma. We want to talk to each other via the net to save money and still have a conversation. But she is a strictly Apple girl, and I'm a Debian man, who compromises with Windows at work. So, does anyone have the solution for an easy cross-platform voice chat application, preferrebly without having to altering my GF's firewall router via the phone?"