I have made a simple web-interface on my server that makes it possible to write and send sms-messages, administer an address book, and see a list of sent messages and delivery reports. The programs are written in Perl and use the services provided my HQsms.com. I live in Denmark so the programs have a Danish interface and use Danish standards for phone numbers and dates. The interface is simple but seems to work quite well.
If my users want to use this service, they provide me the phone number they want to use as "sender number" for their sms-messages. I send this number to HQsms.com, they send an sms message with a code to that number, and the user pass this code back to me. I then register them as users of the service, and they can login using their ordinary user name and password.
If anyone else are interested in a service like this, I will be happy to receive feedback and pointers to similar programs. I can also consider to make an English version or a contrib rpm.