Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
BlockSurvey - Privacy-focused surveys, polls & forms
Ozeki NG SMS Gateway - Ozeki NG SMS Gateway is a powerful, reliable and flexible SMS Gateway application.
Refiner - Customer Feedback Surveys For SaaS
Kannel - Open Source WAP and SMS Gateway
Surveys in Slack by Polly - The world's first fully immersive Slack-based survey product