CroTeam, the makers of the Serious Sam games as well as the Talos Principle, just open sourced the engine behind the earlier Serious Sam games. It’s currently Windows only, require Visual Studio 2013 or later and is under the GPL2 license (blah).
Croteam has released the source code for Serious Engine 1 v.1.10 (a slightly modified version of 1.07 removing some depreciated features and making it easier to compile on modern computers), the engine that powered the classic Serious Sam games, over at GitHub! With it, you can add your own updates to Serious Engine 1 or even make your own executable for a mod project! To get started, use Visual Studio 2013 or 2015 to compile the code. Some features, such as MP3 and OGG playback, along with IFeel support, have been disabled, but can be re-enabled by adding the appropriate dll files. A set of resources from the classic Serious Sam games has been released as well, containing all of the models, textures and sounds used in the classic games. Most of the source is licensed under the GNU GPL v2.
The fact it ships with binaries is pretty impressive and uncommon in these kinds of releases. The GPL2 license however kills any idea of making a commercial project using the engine unless of course you are willing to release all your source code as well.
As mentioned above the code is available on Github.