MAME is seen by many developers to be the foremost emulator of arcade games, and while MAME source code has long been freely available for use, it hasn't technically been open source.
Now, after roughly 10 months of reaching out to people who contributed to the project, MAME is being distributed via the official Mamedev GitHub as open source software under the auspieces of the GPL 2.0+ license.
Speaking to Gamasutra last year, MAME engineer Miodrag Milanovic suggested that allowing people to charge money for ventures involving MAME code would permit MAME to be used in, say, museum exhibits, or retro game compilations.
"There was intention to do this for years," Milanovic told Gamasutra last May. "Our aim is to help legal license owners in distribuiting their games based on MAME platform, and to make MAME become a learning tool for developers working on development boards."