For a few years now I've been using ReStructured Text for nearly all of my documentation needs and I'm loving it.
It was originally invented for Python documentation which is
how I originally discovered it. As devoted fans we use it for pretty
much everything. Documentation, e-mails, even legal documents!
In fact, most of our blog posts, including this one was originally
written as e-mails in ReStructured text which can be automatically
converted into HTML.