Ah, this brings back memories! Taking a page from id Software's playbook, Croteam has just open-sourced the engine it used to make the original Serious Sam and The Second Encounter.
As Croteam itself admits in the announcement, you won't be making cutting-edged titles with the Serious Engine. However, as a learning tool on how code for a 3D first-person shooter is structured, it's not a bad place to start:
"Historically, this version of Serious Engine is very important for Croteam and for me [developer Vyacheslav Nikitenko] personally. I created several mods for Serious Sam back in the day, before even starting the work on the source code, and it was a great tool for learning ... And it's even better today! Obviously, 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.
It's an amazing gesture for the game's 15th anniversary. If you're keen to check it out, the source repository is available on GitHub.
Serious Sam source code released [Croteam]