I have been looking for any and all hardware compatible with ardupilot, and there is a lot but most of it has fell into obscurity simply due to a lack of documentation other than a few forum posts so unless you really know what your looking for and the forum site is still around your probably not going to find it.
Just last week I found there was a whole series of really cheap RTK receivers that had been tested and documented on a forum post on diy drones and a youtube video but there is no mention of its existence in any of our documentation.
I have links for mavlink>RC telemetry converters on github for just about every RC system with telemetry, sure ardupilot supports most of these natively now but they can still be useful for controllers with limited ports or for on UHF relays.
@ppoirier has a whole bunch of proximity projects using various rangefinders and interfaces and have been the basis for a number of my own projects, sure they have forum posts but they should be on the main wiki as they are brilliant. I have a number of sensor adapters and rangefinder projects bookmarked for converting incompatible sensors and have created a few myself.
@Yuri_Rage has his new sensor adapter/IO board that can interface just about anything to Ardupilot over i2c and an esp8266 telemetry radio system.
There are some mavlink led controllers like mavpixel or toshiba LED emulator, mavlink oled screens, mavlink Epaper screens.
There are about half a dozen mavlink antenna tracker systems.
There in an opensource mavlink FLARM receiver.
there are a lot of sensors that can be interfaced as buttons using basic GPIO, water sensors, rain sensors, lightning sensors.
there are a lot of sensors that can be interfaced using the ADC for logging things like water conductivity, turbidity, PH.
il try and put together a more comprehensive list with each devices name and a brief description of it to get added to the wiki.
Im sure there are a lot more compatible arduino projects and lua scripts out there that can be added to an “other devices page” that im missing so post any suggestions and il add it to a pull request to the wiki.