My default for writing anything that is longer than a page is LaTeX (possibly via org-mode, if it is short and simple). In fact, the bond that ties me to the LaTeX/Emacs combo is so strong that I want to use it even for texts that are exactly one page long, i.e. conference posters.
CTAN lists a lot of packages and frameworks for posters, but I found most of them too heavy/complex.
I don’t create a lot of conference posters and did not want to spend ages putting a few words and graphs on a sheet of glossy paper. At the end of the day, I decided to give beamerposter a spin. Beamerposter is an add-on that transforms my favourite presentation package into a poster printing machine. I did not really like the default themes, but Rob Hyndman has created a very alternative nice template that I adapted slightly.
I rather like the result and will go back to the package for the next poster.