https://github.com/x-station/xstation-releases - Optical Disk Emulator; Replace the optical disk with a sd-card reader

https://www.black-dog.tech/ps1digital.html - PS1 Digital - Add HDMI video output to the PlayStation

https://shmups.system11.org/viewtopic.php?p=1341006#p1341006 PU-22 (SCPH-750x) csync mod

http://mgarcia.org/Blog/2019-08-24-PSONE-Serial - Adding SIO to a PSone (SCPH-102)

https://github.com/c0pperdragon/PS1YPbPrAdapter - Sony Playstation YPbPr Adapter

https://github.com/alekexe/Pandora-PS1-Mod - Pandora, an open source ESP8266 modchip for the Sony Playstation 1

https://github.com/kalymos/PsNee - PsNee, an open source stealth modchip for the Sony Playstation 1

https://github.com/ColdHeat/PsNeePy - A Playstation 1 Modchip written in Python

https://github.com/grumpycoders/pcsx-redux/wiki/PIO-port-replacement - PIO port replacement

https://quade.co/ps1-modchip-guide/ - PlayStation 1 modchip installation guides