SUMMARY
When it rains, it pours. The game engine that powered the games "Serious Sam: The First Encounter" & "Serious Sam: The Second Encounter" (otherwise referred to by the developers as Serious Engine v1.10) has now been open-sourced in celebration of Serious Sam's 15th birthday (time flies when you're having fun). Here's your chance to take a peek under the hood at what powered what many considered as a spiritual successor of the run-and-gun FPS gameplay that made Doom I & II so popular. Enjoy.
LINKS
GitHub repository for the Serious Engine source code
The announcement from Croteam that they have open sourced the Serious Engine