Hey there. As skype is closed source, i searched for an alternative beeing open source, nearly as powerfull as skype and runs on linux as well as windows. I tried gismo but figured out soon, that it isn't as stable as skype.
Now i found wengo and wonder if it would be possible to built a portable version of 2.0 as there already is a portable version of 0.99 (which can be found in forums around there).
I think wengo would be a good alternative to skype and its published under GPL, so there shouldn't be any issues concerning licences. See http://www.openwengo.org/