KOF 91 (aka KOF Linux or FFE) is a 2D Game Engine like Street Fighter but everything is easily customizable so you can easily add characters & Backgrounds, etc or even make your own game. Read the docs for more precisions.
KOF 91 was first developed by Moah on Win32 in the early 98 (see http://www.kof91.com ). The linux version was developed by Duncan Mac-Vicar P. and Ric's Brain (who add patches before). There's a GP32 port made by Thor ( http://www.geocities.com/rtb7 ). Ric Leite still develop new features, under KOF2003 project (see centurion forces site for news) and a new OS independant version is still in developement (2004) by Laurent Dufour (zesensei) with new features.