In the Python world, readthedocs.org has made writing docs easier - it takes care of rebuilding documentation each time you commit. It's implemented in Python, and most popular with the Python community, but it can be used for other languages as well.

Sphinx is pretty nice, even if it is not a python project.

http://sphinx.pocoo.org/

API documentation can be generated from the source code with tools like doxygen. For everything else there's the standard toolchain consisting of a good editor and source control. Writing documentation is a chore but it's a very important aspect nevertheless.

I'm a marketing and technical writer - take a look at the URL in my profile and get in touch if you are interested.