Paintown is an opensource 2d sidescrolling beatemup fighting game + engine.
It allows user custom enhancements as well, through scripting and supports Mugen themes, stages and characters.
This makes Paintown an open source alternative to Mugen, maybe better because it supports online play as well.
Freebsd Port: www/games/paintown
Note: The port is out of date, the newest version is 3.5. I asked the paintown community to contact the maintainer of the port. Hopefully, the port will be updated.