For the 15th Anniversary of Serious Sam Croteam released source code for Serious Engine v1.10! It’s the very same engine used for Serious Sam Classic: The First Encounter and The Second Encounter. Serious Engine v1.10 won’t produce top-notch graphics, but the source code is very well commented, easy to modify, and there are lots of user generated mods out there. This version has everything you need to build your own game – or just experiment. A Linux source code release is in the works too.
Link to Official Site (Source Code)
Link to Official Site (Comunity and Tutorials)