Update (2/22/2021 9:53 AM) - Take-Two Interactive, the parent company of Rockstar Games, have issued a DMCA takedown to github, removing the reverse-engineered games from the site. However, it's likely that the code is still out there on smaller sites and torrents.
The original story follows below
A team of modders has announced that they have successfully managed to reverse engineer the code for Grand Theft Auto 3 and Vice City. This effort, which took almost three years of work, means that anyone can examine or play around with the code for these two landmark video games.
Announced yesterday by a Reddit user known as erorcun, the project (re3) was started in the Spring of 2018 by kotzkroete. About a year later, the project moved over to GitHub and a few more people joined the team. From there, they started work on the Vice City code (reVC) in May 2020 by starting off from Grand Theft Auto 3's code, and considered it finished at the end of the year.
What does this all mean for the players?
One of the major effects of reverse-engineering the code is that it allows the addition of a variety of quality-of-life upgrades. Just a few of these include shorter loading times, the ability to rotate your camera while in a vehicle (a feature introduced in San Andreas,) new languages for Grand Theft Auto 3, and a bunch of new cheat codes. A full list of what's been changed with the games' code can be found on the project's GitHub page.
Another big consideration about this work is that it allows both games to be ported to other platforms. So far, the GitHub page states that the game has been ported to the Nintendo Switch, the PlayStation Vita, and the Wii U. Speaking of consoles, the code requires the PC version of the game for assets. Interestingly, the reverse-engineered code does not work on either the PlayStation 2 or Xbox, meaning there's probably some small differences from the original or some missing special sauce given that the games were first released on PlayStation 2.
The code for both games can be found here. It's also worth checking the Reddit post to learn more about getting it working.