Cross-Platform VoIP Software?

feilkin writes "With the release of Skype's Linux client, I'm wondering about alternatives. Namely, cross-platform solutions for voice communication. I've got friends who are using Windows, Linux and OSX, and I'm hoping that there is a way to communicate with all of them. I myself am using Linux, and I haven't been able to find any solutions that seem fitting to my situation completely. Does anyone have a solution that'll be useful on all three platforms, or solutions that may be coming in the near future?"