Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Dingtone - Dingtone is a free call and texting application for making unlimited calls and text messages to any number at the moment registered in any part of the world by any cellular company.
Sipdroid - Free and open-source SIP/VoIP Client
Textnow.com - It lets you text your friends, aka my only source of happiness
Sipnetic - Sipnetic is a free VoIP softphone based on the SIP protocol.
FluffyChat - Open. Nonprofit. Cute ?