SMS Gateway Suggestions Needed
There are a number of companies that have SMS service, allowing you to make an API call to their system and they then deliver the message. They take care of all of the relationships with the various service providers, which can be a very time consuming process especially if you're looking at delivering messages interationally.
We've worked with Twilio, Clickatell and BulkSMS for the past few years, their coverage and rates vary but they're all easy to integrate with, usually only a few lines of code is enough to get started with their APIs. The rates depend on what network you're sending to, but typically are between $0.02 and $0.15 per message.