I have been working on PlayStation 1 homebrew SDKs [1] and tools over the last year. Unlike other retro consoles - especially Nintendo consoles - the PS1 gets almost no attention at all; "PS1-style" games made using Unity or Unreal Engine seem to be decently popular, but nobody wants to bother with the real hardware anymore. Which is a shame, as the PS1 is a relatively simple platform which can be found for relatively cheap and offers a bare metal development experience reminiscent of modern 32-bit microcontrollers, with some interesting graphics and audio hardware thrown in. It is a platform unencumbered by the tile/sprite and storage limitations of the 8- and 16-bit eras, yet still limited enough to be a breeding ground for creativity and at the same time easily expandable with custom hardware through its integrated serial port and ISA-like parallel bus.
I still have a Net Yaroze sitting in a box in my garage, including all the manuals and CodeWarrior that probably won't work on anything anymore. I'd love to donate to someone who would use it, but it literally has a lifetime non-transferrable license :(
Most of the SDK has been tested and confirmed to be fully functional on real hardware, but there is some copy protection you will have to get around in order to run arbitrary software on a PS1. The traditional solution is to perform disc swapping or install a modchip, but nowadays we also have softmods [1] that make the process as easy as inserting a specially formatted memory card (which can be prepared using a PS2, or even a PS1 by swapping discs).
You are not required to use CDs either. With the help of some code and linker script magic, you can build an image that can be booted directly from a cheat cartridge (or simply a parallel EEPROM) connected to the console's expansion bus, bypassing the copy protection checks entirely. There are also debugging tools that, once loaded, let you download an executable into RAM for quick testing and manage memory cards using a modified serial cable [2].
[1] https://github.com/Lameguy64/PSn00bSDK