What are some good examples or paradigms for Client and Server implementation in the same source file?

Question

Sometimes for small projects, and even not so small ones, its nice to be able to keep server-side implementation and client-side implementations on the same source file to reduce maintenance overhead, and make it easier to quickly develop stuff.

So I was wondering about good sources of documentation and frameworks that do this sort of thing.

Note: I'm implementing a small client-server framework and want to make it easy to prototype, so I want to reduce the amount of files and overhead of starting new projects.

Thanks