Team Fortress 2 (partially found original builds of PC multiplayer first-person shooter sequel; 1999-2006)
Team Fortress 2 is a free-to-play first-person shooter game, being one of the most popular games on Steam. The game was originally going to be much different to the product we have today. The game was going to share a similar art style to the original Team Fortress, and was to be dubbed "Brotherhood of Arms". The game was first announced in E3 1999.
The game was delayed twice, until it simply slipped off the radar in 2000, and resurfaced in 2006, as a vastly different game, with a cartoon-like art style, as opposed to the original, realistic design. Valve revealed that they had tried "3 to 4 designs", before settling on the final design.
These other builds have never surfaced, and only the first was ever publically showcased. Two models of the build were found in Half Life 2's code, and could be accessed via cheats. This was revealed to be a different build, which would have NPCs present during multiplayer matches, futuristic weapons, and a possibility of the game being based in Half Life 2's universe. These were part of the second phase of development.
The other builds remain a mystery.