Spranto

http://www.spranto.com

There are two versions, Lite (Free) or Profesional (Commercial).

Yate client

http://yateclient.yate.ro/

Free

Mirial Softphone

http://www.mirial.com/

Commercial

PacPhone

http://www.pacphone.com/

 

NetMeeting

http://www.microsoft.com/downloads/en/details.aspx?FamilyID=26c9da7c-f778-4422-a6f4-efb8abba021e&displaylang=en

 

Ekiga

http://ekiga.org/