A fan called GalaxyHaxz has managed to reverse engineer the source code for the original Diablo and released it on Github. This is not a free version of the game, it includes no game assets, but it will make it more accessible for modders to update, fix and change.
The project goes by the name of Devilution and the purpose of its release is as follows:
Having the source code makes things much easier to maintain. For years mod-makers had to rely on tedious code editing and memory injection. A few even went even further and reversed a good chunk of the game (such as Belzebub/The Hell). The problem is that they never released their sources. Usually being a one-man job, they move on with their lives inevitably due to the amount of time/work required or lack of interest. This leaves people with a half-finished mod; one which had countless hours put into it, but left full of bugs and unfinished potential. So we’re back to square one. Devilution aims to fix this, by making the source code of Diablo freely available to all.
The goal of Devilution itself is to recreate the original source code as accurately as possible, in order to ensure that everything is preserved. This goes as far as bugs and badly written code in the original game. However, it becomes a solid base for developers to work with; making it much easier than before to update, fix, and port the game to other platforms.
As a side goal, Devilution helps document the unused and cut content from the final game. Development of Diablo was rushed near the end–many ideas were scrapped and Multiplayer was quickly hacked in. By examining the source, we can see various quirks of planned development.
More than 1200 hours have been poured into making this release and the plan was to create a Linux port, convert to OpenGL, and modernise the UI. It’s an interesting project and you can find out more here.