Git

Yeah. Seeing the development thought process at work during the engineering of git was really cool. The philosophy was basically, at its core it's not a version control system. It's a content-addressable filesystem. That's what you need in order to build a good distributed version control, so we'll make two layers and make each one individually very good at what it does. Then in a UI sense, the idea was to give you the tools to be able to do needed operations easily, but still expose the underlying stuff if you need direct access to it. And then to optimize the whole thing to within an inch of its life under the types of workloads it'll probably be experiencing when being used as version control.

It was also, as far as I'm aware, the first nontrivial use of something like a blockchain. The property where each commit is referred to by its hash, and the hash encompasses the hash of the previous commit, was a necessary step for security and an obvious-in-retrospect way to identify commits in a unique way.

Basically the combination of innovative design with a bunch of core concepts that weren't really commonly in use at the time, combined with excellent engineering to make it all solid and working well, was pretty mind-blowing to see, and it all came together in just a few weeks and started to get used for real in a big sense. Then, the revolution accomplished, Linus handed git off to someone else and everyone just got back to work on the kernel.