Pie in the Sky engine and the Colosso series
Pie in the Sky engine was a game engine developed by Pie in the Sky Software. The Pie in the Sky engine was also used by independent companies to develope their games.
The Colosso games (three FPS part of the same series) are quite rare and, surprisingly, the developer liberated all three of them and they are available on the official site to download. The official site itself is a rarity. I don't think it changes one bit since the 90s.