Clockwork is quite possibly the best SMS API out there. It's unique, it's intuitive, it's simple, it's easy to use, and above all it works. Clockwork has some the best customer service I've ever seen. Their twitter presence is a shining example of how other companies should manage their social media. Others should strive to be as good as Clockwork in this regard. The fact they use a singular get request (Get!) To both send the data of the SMS and receive whatever message the API returns is simply marvellous. Literally. The simplicity and intuitive nature of this method opens up SMS marketing etc. to a range of users. Not to mention their wide range of wrappers, the ability to be used in almost any language, and the extensive yet concise documentation that they provide. Clockwork is everything that an API should be.