We recently worked with two different clients on Text Messaging (SMS) reminders for upcoming appointments. One client was in Real Estate, and scheduled home showings with prospective buyers. The other client was in the health and beauty industry, and sent appointment reminders for spa treatments.
In the real estate example, the text messages needed complex data and rules that exceeded some off-the-shelf products. We elected to use Twilio, an SMS provider with excellent Salesforce support. Using their API, we were able to implement this feature in a very short period of time.
In the health and beauty project, the text messages were less complex. We elected to use SMS Magic. This app provides Salesforce support right out of the box. A large percentage of clients prefer these types of text messages, so this was an excellent fit for their business.