After 19 years, MAME is now available under an OSI-compliant and FSF-approved license! Many thanks to all of the contributors who helped this to go as smoothly as possible!
We have spent the last 10 months trying to contact all people that contributed to MAME as developers and external contributors and get information about desired license. We had limited choice to 3 that people already had dual-license MAME code with.
As a result, a great majority of files (over 90% including core files) are available under the 3-Clause BSD License but project as a whole is distributed under the terms of the GNU General Public License, version 2 or later (GPL-2.0+), since it contains code made available under multiple GPL-compatible licenses.