There's a whole load of 2.5D FPS games made using the
Pie in the Sky engine (also known as 3D Game Creation System). They're of varying quality, but the engine was rather popular with some people until late-nineties, presumably because of the ability to create a game yourself.
Original games by
Pie in the Sky Software:
Corncob 3-D: The Other Worlds Campaign (1992)
Lethal Tender (1993)
Titles by
Acord Games:
Zoom (1994)
Cavemaze 3D (1995)
Lost in Zaperstein (1995)
Wicked Dream (1995)
The Colosso Adventures series:
Episode 1 Shareware v1.4
Episode 2 Shareware v1.2
Episode 3 Shareware v1.0
Other games:
La Cosa Nostra (1994)
Red Babe (1994)
Castaway: The Ordeal Begins (1995)
Deer Napped (1995)
Space Station Escape (1995)
Terror in Christmas Town (1995)
Troubled Tower (1995)
Escape POW (1996)
Gaia (1996)
Master Won Liu 3D (1996)
Ultimate (1996)
Zone Force (1996)
CHUB GAM 3-D (1997)
Revenge of Dr. Chang 3D (1997)
Chub Gam 3-D: Director's Cut (1998)
Pencil Whipped (2000)
[Edit] Even more games are listed
here (DOS) and
here (Win9x)