Open source Skype application
Skype uses proprietary protocol and there are no opensource application which can make use of this protocol. However skype is planning to opensource the skype application (protocol will still remain proprietary) however they havent specified a time frame yet.
There is no open source Skype application.
But of course there are several alternative VoIP applications that use the standard SIP protocol (including Empathy and Ekiga) which is supported by most VoIP providers (except Skype...).