Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
TextMe - TextMe is a free calls and messaging application at both local and international levels.
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
Talkatone - Android App on. Google Play. Stop paying for cell minutes, texts & landlines; Turn your iPad or iPod touch into a phone; Call/Text even in areas with bad cell coverage; Call/Text when travelling without paying for expensive international minutes.