Gizmo CallOut is an open source, open standards, generally open alternative to Skype. Ironically, unlike Skype, Gizmo does not have a working Linux version, but they promise to in August.
Looks like Skype’s rates are slightly better. I don’t know much about Skype despite having downloaded it and played with it once, so I can’t say overall whether Gizmo is that much better, worse, or whatever, but it has features Skype denies you, like the ability to easily record a conversation… cf. Scott Kurtz having to go through considerable contortions to record Skype calls.