SwissSMS

SwissSMS is a Mac OS X front-end to SMS operators services.
It stores your outgoing messages and can export them too.
If you can code, then you can easily add support for your own operator!

See the API and the plugin template.
Main features:
- Address Book integration
- an Address Book plugin
- an Automator Action
- a Spotlight importer
- a command-line mode (think sending sms from your scripts)