The Croteam website has word that Croteam has released the source code to the Serious Engine version 1.10, which powered the original Serious Sam games (thanks Seriously!). All the files are posted on GitHub, and here's the explanation of the release:

Hey guys, today we have a surprise for you. We’re releasing the source code for Serious Engine v1.10! It’s the very same engine that we used for Serious Sam Classic: The First Encounter and The Second Encounter.

This month we’ll be celebrating Serious Sam’s 15th anniversary and we’re all feeling very nostalgic. Croteam’s Vyacheslav Nikitenko, who worked on the source code and prepared Serious Engine v.1.10 for this release, had this to say:

“Historically, this version of Serious Engine is very important for Croteam and for me 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.

If you’re looking to get started, just download the files from GitHub and head over to SeriousZone, it has a great community and lots of tutorials.”