Probably the world's most interesting operating system. Some quotes from the CodersNotes blog on the OS:

TempleOS is somewhat of a legend in the operating system community. Its sole author, Terry A. Davis, has spent the past 12 years attempting to create a new operating from scratch. Terry explains that God has instructed him to construct a temple, a 640x480 covenant of perfection. Unfortunately Terry also suffers from schizophrenia, and has a tendency to appear on various programming forums with a burst of strange, paranoid, and often racist comments.
TempleOS has its own programming language, HolyC. The whole operating system is written in it, except for x64 assembly in the lower-level parts. Perhaps unexpectedly, the same language is also used for the shell. That's right, you execute shell commands using a C-like language, and they go directly into the compiler.

HolyC seems to be very similar to C, but it has something akin to a REPL. So you can run int a = myfunction(); without all the boiler plate required to create an executable and compile it in C. That's a pretty incredible achievement, and something I have wished C/C++ had for a long time!

Every piece of code in TempleOS (except the initial kernel/compiler) is JIT compiled on demand. Yes that's right - you can run a program without compiling it, simply by using an #include statement from the command line. The program is then brought into the shell's current namespace, and from there you can execute individual functions directly just by issuing commands.

The offical TempleOS website can be found here. An ISO of the OS seems to still be available for download, although its creator unfortunately passed way in 2018.