Most people don't know about it anymore these days, but Serious Sam was developed as a tech demo for Croteam's Serious Engine. Hence why you also have a demo level which showcases all the tech in the game. Even the Karnak temple demo was designed to showcase as much of the capabilities of the Serious Engine, especially during the intro, which shows you most of the level (minus most of it's secrets and later spawning-in ennemies).
The Demo was so well accepted by gamers that instead of selling the engine, they ended up selling a ton of games
Edit: Also memorable: NVidia's Dusk and Dawn demos. Most memorable because they ran better on Radeon GPUs than on NVidia's own GeForce FX...
NVidia remade that last one later on for more modern GPUs btw:
Last edited by Bofferbrauer2 - on 14 February 2021