OpenBOR is a free and open-source side scrolling game engine. There are hundreds of fangames of popular franchies created for OpenBOR. It's mostly optimized for side scrolling beat 'em up style games (Double Dragon, Streets of Rage, Final Fight), but can easily accommodate any sort of gameplay style and feature.
OpenBOR is based on the freeware DOS title Beats of Rage released in 2004, a beat 'em up game heavily inspired from Sega's Streets of Rage series. Beats of Rage's source code was released publically in 2005 and OpenBOR was born.
Many games are available on ChronoCrash Modules Forum. OpenBOR might be included with game files.