I use RST (reStructured Text) and VIM. If necessary and for collaboration, I convert it using docutils (python tool) to html, rtf, pdf, latex, or whatever. I also write e-courses in RST and have sphinx (python doc tool) generate the whole html page for me. So for me, RST is extremely versatile.
I hate really **hate** being reliant on legacy software AND not being able to open what is basically text with some basic formatting in a text editor or in the terminal.
For my work, I've had multiple cases where we simply didn't had the software lying around to open legacy docs. That made me switch to RST for once and for good.
For heavier formatting, I just use LaTeX. But that is too much hassle for your needs I guess.