Years before Halo: Combat Evolved became the next evolution of first-person shooters, it was actually an entirely different game. Believe it or not, developer Bungie planned it to be a third-person shooter exclusively for Mac computers, and it went through tons of design changes prior to finally hitting the market. For some random reason, the original beta builds have suddenly appeared on the Internet and are fully playable on a modern PC.
Spartans in 1999 looked quite different. Interestingly enough, 343 Studios has been working behind the scenes to bring special levels to Halo: The Master Chief Collection, including the infamous Halo 2 E3 Levels.
You can play the pre-Xbox Halo build for free right now, as it’s available for download on Archive.org. This features the original, official beta builds for third-person Halo and works on modern hardware. As the Archive.org piece mentions, run blambeta.exe and blam.exe respectively for each specific beta testing level.
In addition, the 1999 pre-Xbox Halo build has dumped source code information for nearly every other release in the series since.