Its almost 20 years since Nicola kicked of mame. The open source thing is just a legal technicality. The source code has always been there for anyone who is interested to view and/or to contribute to.
There must have been hundred, if not thousands of contributers over the years. Would have taken a while to track them all down.
Should be in the next Civ game as a Wonder of the digital age.
PMB / Shodan