Twilio - Brings voice and messaging to your web and mobile applications.
MailChimp - MailChimp is the best way to design, send, and share email newsletters.
Clockwork SMS - Clockwork is an Easy Text Message API. An SMS API for Developers. Build powerful apps and include SMS. Signup is free, Try it now.
Brevo - Innovative online Email Marketing solution to manage your contacts, create & send your newsletters and track your results. More than 80 000 clients. Best prices and attractive features.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Campaign Monitor - Email marketing software built for designers and their clients to run successful email campaigns.