Out of curiousity, what is your favorite system, and why?
I've noticed on many PbP Boards that there is, suprisingly and much to my delight, many varied systems, old and new, available to play.
I'm glad, personally, that the whole world didn't put some of the older, and less popular systems, in the closet when d20 came out.
I prefer d20 Modern, 1E Gamma World and Basic ODnD Myself.
Especially Basic DnD, because it provides just enough structure to resolve and stat things out, yet leaves much more to the imagination and to the story development. And you can learn it in about 2 minutes.