Looking for a good entrance point to client-server applications

Question

I am looking for a recommendation for a good entrance point for building a client-server application.

The target developers will be students in a course (with at least basic knowledge and experience in programing but not necessarily in the client-server world).

Any suggestions will be appreciated.
A suggestion could be a platform (e.g. node.js, google app engine), programming language, framework, enviorment or anything else that you can think of.

Preferably, I am looking for the following:

• Should work in linux environment.
• Easy to learn.
• Widely used (for available examples online...).
• Open source.
• Allows to quickly build a prototype.

Answer 1

Eventually I went with Play Framework.
As I needed:

• It works in a linux environment.
• Is easy to learn.
• Has great manual and examples.
• Is open source.
• Allows to quickly build a prototype from a template or using a manual