Sphinx is very nice to work with, and it's been around for a long time. I've used it for Python projects, C++ projects (with doxygen and breathe), and some programming language agnostic things. It's almost trivial to pair Sphinx docs with readthedocs.org as well. For smaller pieces of documentation (and many personal things) I use Org mode in Emacs. Org mode supports exporting to a wide variety of formats.
We use Sphinx extensively in our organization, as it's text based (works nicely with GIT) and extensible (we can add the necessary directives to support domain specific documentation requirements from CENELEC standards).
Our product is a railway signalling onboard system.
