TextNow - TextNow is a mobile virtual network operator (MVNO) that provides a free phone service to users by using other cellular networks coverage.
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).
Burner - Disposable phone numbers -- now with MMS photo support
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.