What Python framework for a REST/JSON web service with no front end?
At Pycon Australia, Richard Jones compared the most popular lightweight web frameworks. Bottle came out on top. Here is the full presentation.
In general, I think you'll find web2py to be one of the easiest frameworks to set up, learn, and use. web2py makes it very easy to generate JSON (just add a .json extension), and it now includes new functionality to automatically create RESTful web services to access database models. In particular, check out the parse_as_rest and smart_query functionality.
If you need any help, ask on the mailing list.