Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
Twilio - Brings voice and messaging to your web and mobile applications.
TextMe - TextMe is a free calls and messaging application at both local and international levels.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
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.
BulkSMS - SMS messaging service provider offering two-way SMS communication services.