Melrose Labs SMSC Simulator - SMPP SMSC simulator to test SMS messaging applications.
Tatango - Tatango is a mobile messaging platform for agencies.
Tropo - Tropo is a cloud API for voice and messaging.
Postscript - Like Mailchimp for SMS
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.
Flowroute - Flowroute provides direct access to telephony resources - such as calling, messaging (SMS & MMS), call routing, SIP Trunking and Communication APIs.