This is a bit of a surprise! Someone going by the name 'GalaXyHaXz' has put up source code for the original Diablo that was apparently reverse engineered called 'Devilution'. It doesn't have the data, so you do need a copy of the original to work it.
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.
I don't know enough about that myself to say if it's legit, legal or anything, but it's still rather interesting. It's especially fun, since in the FAQ, the developer noted they plan a separate project to update it with a Linux port with OpenGL, modernize the UI and much more.
On the project page, the developer notes that it can apparently run in Wine on Linux right now.
It will be interesting to see what Blizzard do as a response to this. Find it on GitHub.
Thanks for the tip Roney.