Zoiper is a free softphone that can be ran on multiple platforms.
Zoiper can be setup for the use of SIP, IAX or XMPP. To download Zoiper click Here. This link will provide a link to the operating systems available. There are multiple versions of Zoiper available both free and Paid. The newest version includes multiple skins, and revamped settings pages. If you chose to download the free version, you will still be available to upgrade there is a direct link inside the Premium tab.