There is now a section on the Banana for unfinished and finished map source files called "MiPs" (Maps in Progress). The MiPs section is like the prefabs section, but it's for entire maps rather than individual map objects.

It's primarily intended for incomplete maps that mappers have chosen to pass on to the general public, to allow other mappers to take up the challenge of finishing. It's also for open source files of completed maps, to allow new mappers to learn from the pros, or to allow others to remake, extend or mash up existing classics.

Check it out and if you're a mapper yourself, please consider adding some of your own map source files for other mappers to work on and learn from.