Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
TextMe - TextMe is a free calls and messaging application at both local and international levels.
Kannel - Open Source WAP and SMS Gateway
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.
Ozeki NG SMS Gateway - Ozeki NG SMS Gateway is a powerful, reliable and flexible SMS Gateway application.