I found out about Mugen 2005. I remember seeing Super Mario fighting Evil Ryu, and I was surprised such a thing was possible. I didn't look into it, though.
I downloaded Mugen for the first time in 2011 (strangely enough, it was the same day 1.0 was released). I don't know what caused me to remember that video I saw suddenly, but I thought maybe I should check it out.