Oh there's soooo many, I'll copy, paste and add to my list from the original thread:
Bouncer - the holiest of lost game holy grails
Fire & Forget II - Titus Software, cab on flyer looks suspiciously like a modifed After Burner cab
NY Warriors , Aaargh! and Rockford - Arcadia games ported to various home computer platforms, nobody is quite sure if they were ported to the Arcadia Super-System arcade boardset like other games.
Angler Dangler - Data East / a MAME dumper may have finally found a copy of the DECO tape, featured on Star Cade and ported to the TI 99/4a home computer.
Missile Command II - Simutanious two-player version, not the classics multi-PCB version. One working boardset happily in the hands of a KLOVer!
Black Widow - early Taito raster version, supposedly a complete boardset is in the hands of a KLOVer's friend. Uses the same boardset as Qix
Surfer - Only a vague flyer shows any proof of its existance
Atomic Castle - lost laser disc game, supposedly only one ever made. I remember an article showing pictures of it in a game magazine and it looked pretty cool.
Vertigo - Vector predecessor to Tunnel Hunt. Supposedly a complete example still exists in the hands of an Exidy programmer
Ms. Gorf - Programmed but never committed to hardware, supposedly still exists as uncompiled prototype software on 8" floppies
Journey - Stern version, not the Midway game. Was shown in the background on Star Cade.
Break Away - Venture Line Breakout rip off
Crazy Otto - Ms. Pacman proto, ROMs supposed still exists in the hands of its programmer whom is reluctant to release it due to copyright issues
Battle Star - lost Sega vector
Cutey Fatty - Funny and fun looking Korean game about fighting obesity
The Last Star Fighter - supposedly made it partially through the prototype stage, there's a Youtube video supposedly showing what exists of it.
Zool - Predecessor to Sonic the Hedge Hog, pictures of the cab and PCB exist, was ported to at least the PC
Kreepy Krawlers - according to the Exidy programmer it was completed but never released, no one knows if the ROMs survived.
Ms. Pacman Twin - Simutanious two-player MP knock-off. Supposedly one working example has been found in the wild and partially ROM dumped.
Hovercraft - Lost vector game that might not have made it out of proto
I'm pretty sure I saw one on Ebay within the past year and a PCB on KLOV more recently, the prototype Jungle Boy might qualify better.