Just getting round to checking out M.U.G.E.N. LAst time I looked I couldn't find a simple download so got bored looking and gave up on it. Just found a great site to get started. It has downloads for M.U.G.E.N. itself as well as DC and Marvel characters and backgrounds for download.
You can add limitless characters that have been ported by people that are available to download. All sorts of strange combinations can be had (Homer vs Batman) but it does kinda make sense to keep it a bit more simple than say...
When I've got this installed and working i'll post some more info on what you can do/need to do and where to get more characters.