Now available under the GNU GPLv2 license is the Serious Engine, albeit the older v1.10 engine. This open-source Serious Engine so far has been tested on Windows but it would not be surprising to see developers in the community working on the Linux build soon enough. The assets from the original Serious Sam games like Serious Sam Classic: The First Encounter should work with the open-source engine.
Those interested in the open-source Serious Engine game engine for reliving the older Serious Sam games, the GPL code can be found on GitHub. The official announcement was posted today to Croteam.com.