An original engine for Serious Sam Classic games (source files)
serious_sam_source-1.10~git-4-source
The Serious Engine lets you run the Serious Sam Classic: The First Encounter and the Serious Sam Classic: The Second Encounter games. In addition, custom game mods are supported also. Croteam pledged Serious Engine to go open source under the GPLv2 on March 11, 2016.