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 announced in 1999 and 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.
Brotherhood of Arms (1999)
The first build of 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.
This version would've been significantly different from previous Team fortress games. The gameplay, maps and weapons were going to take a more realistic approach, and there were going to introduce 12 classes, rather than the 9 classes from the final game.
Invasion (2003)
The second build of the game was leaked in 2003. Unlike the previous build, this version was less realistic and would probably take place in the Half-Life universe. Two models of the build were found in Half Life 2's code, and could be accessed via cheats. This build would have NPCs present during multiplayer matches, futuristic weapons. The classes would have ben the same as its counterparts', except for the Demoman class, which would have been replaced with a "sapper" class. Instead of red and blue teams, there would have been an alien team and a human team, all with different models for each team.
Final design (2006)
Team Fortress 2 was re-announced in 2006. In the July of the same year, Electronic Arts and Valve Software revealed the "final design" of the video game "Team Fortress 2". The conference and Valve screened 2 teaser trailers demonstrating the game. The trailers showed a build of the game that was similar to the original but had some differences in it. This includes weapons from the original Team Fortress Classic game; Near finished designs of the characters with things that weren't in the final design of the game (examples like the Spy and 2Fort) and gameplay as well.
A few things from the build is in the current version of the game, but however, things like the models and the map didn't make it and aren't public. A few people in the Team Fortress 2 community try to recreate these models and the map for the current version of TF2. The only remaining evidence of this build are the trailers from the conference itself and pre-release screenshots that appear on most gaming sites.
The other builds remain a mystery.