Hi all, just in case there is still someone with german provider o2 and post paid contract around AND using its N900: Here is an updated provider file for O2 Germany, based on the last version linked in this thread.

www2sms_providers_web.py.zip

On your N900, replace file
with the one from the zip-file (consider making a backup before), then start www2sms app and choose new provider "O2 Germany App2SMS 2014" . Login username is plain phone number .

Note: Some weeks ago O2 Germany has changed their webmailer and - along with that - stopped offering a service for sending sms with a webbrowser. Instead they added that possibility into their app "Mein o2", only available for Android and iOS. So I converted the requests and responses (with required data) from that app into a new www2sms provider class.
The app description in google play store states "50 free sms per month for customer with postpaid contract". Thats true for me at least, but might require a renew of login credentials for other O2 customers ...

Note2: The self update function of www2sms doesn't work anymore. It was looking for http://olausson.de/downloads/view.download/2/2 , which asks for a login by now.
Anyway, thanks to Björn/blubbi for www2sms! I am happy to contribute a bit ...

Cheers, Ulle