here it is! because I also want to use my GMX free sms
feel free to use and test it, and please report bugs!

it wasn't easy to hack this together, there are nearly no docs for python plugins for Web2SMS...
but with the SIPgate plugin from this thread and the source from the project I finally got it working.
Thx @Web2sms-developers, it's a great app
The sms sending functionality is from http://gmxsms.sourceforge.net/ with some modifications... (Thx for that project, because GMX-SMS hast no API docs...)

Features:

• Send SMS via GMX
• Show Account Balance
• 'Session handling' for the GMX stuff
• Full Error handling (?)

ToDo:

• Get this unicode stuff working (e.g. '€' is delivered as '?')
• Multiple Receipients (implement and test)
• Display costs if no more free sms available
• fix that ugly code

You don't need your GMX password! There is a 'Freischaltcode' on the GMX page:
Login to your GMX account, navigate to "SMS und MMS" and then choose "Download und weitere Informationen", on the bottom of the page you'll finde the code.

to use it, remove the .txt ending and save it in /opt/web2sms/providers/

If somebody uses this as example for other python plugins: forget the SMSManager class, it is for the GMX-related stuff...

[EDIT] updated provider plugin, fixed some bugs with umlauts [/EDIT]