Croteam has Open-Sourced The Serious Engine

Croteam has Open-Sourced The Serious Engine of thier classic Serious Sam games that were released back in 2001-2002.

The engine was quite impressive at that time, the graphic part (supported both D3D and OGL) and multiplayer, mods, etc, i remember playing those and was lots of fun :)

Grab the source code: https://github.com/Croteam-official/Serious-Engine

Btw, those games are now part of Serious Sam Classics: Revolution bundle (pretty cheap): http://store.steampowered.com/app/227780/