Ekiga (http://ekiga.org/), at least, is both free software (by definition, not free to download binaries) which runs on at least windows and unix-like OSes, supports audio and video connections, works over open standards (SIP and H.323) and even lets you get a free SIP account at ekiga.net.
The uniqueness of Skype is overstated, the principal advantage is the same that using Facebook has over using any open standard social networking, everyone you want to contact is on it. So Skype and Facebook are a good match, both are walled gardens with limited (or no) interoperability or support for open standards.