Documentation is good. Tests are good. If they are done right, it's even better. But hardly anyone wants to spend a lot of time writing tests or documentation, so even just some people documenting and testing bits of the same code will make it better than having everyone go out and re-invent the wheel.

Make sure you make it easy to adapt, test and document the code. That's where revision-control comes into play. I'm using CVS at the moment for most of my projects, but subversion is looking to be the best thing since CVS. I've only used it on 2 projects, yet, but as soon as I've got the time, I'm planning on switching my CVS repository to subversion.

I would also recommend using some sort of Wiki for "general" documentation (introduction to a module, giving an overview of the whole code library/project) - just because it's easy to change will mean that people will actually update the docs. Remember, programmers should be lazy :-) If it's easier to write a new plain text-file in some obscure subdirectory than to update a word-document in the project-document system, guess what they'll do. Avoid wordprocessors for documentation as much as you can. Programmers hate them: they're big, slow and you can't write code in them. A good Wiki is much easier.

updated: spelling.