OpenBOR is the open source continuation of Beats of Rage, a Streets of Rage
tribute game originally developed by Senile Team. In the years since it has
grown into arguably the single most powerful two dimensional sprite-based
engine in the world and certainly the most versatile. While intended to allow
creation of side scrolling beat 'em ups like Double Dragon or Final Fight, the
engine is also capable of replicating shoot 'em ups, RPGs, platformers and the
like. A built-in scripting engine allows authors to take it even further,
making imagination the only real limit.

The games come separated in .pak files: you need to download them, extract
them and put them under the the Paks folder (~/.openbor/Paks/).