Some time ago I took part in an SMS-related website project, which did not materialize in the end. As a result I have been left with my working implementation of MXtelecom's SMS gateway HTTP interface in form of a Drupal module.
Technical details about what exactly the module does can be found at MXtelecom's website: http://www.mxtelecom.com/uk/tech/sms/http
The module is merely an implementation of the HTTP interface protocol described there, respective configuration admin pages, plus the underlying DB structure to store data about sent and received messages and delivery reports.
I didn't really have a chance to look at the existing implementations, apart from trying to find one that would already have support for MX Telecom SMS gateway. Which none of them did.
There's a couple of framework modules, though, on top of which I was considering to build my module. The most promising of them is SMS Framework. Eventually I decided to go for a stand-alone module, as there would have been no easy way to get the MX Telecom's message delivery report working or to send/receive any message types except for the plain SMS. I'd like to have a working implementation of the full MX Telecom SMS gateway HTTP protocol, including the model specific messages (config, logo, ringtone) and MMS.