This comes up rather frequently on this forum. You are the first person who has done more than just whine that such a web site doesn’t exist and actually try to build one. I wish you all the luck.
But in my experience, such websites end up being out of date pretty much as soon as it is created and will never be complete. I don’t know anything about Home Assistant, but OH has support for around 350 technologies and APIs. Each one potentially supports hundreds or thousands of devices (e.g. Zwave, Zigbee, Knx). Some are general purpose like HTTP, Exec, TCP/UDB, MQTT, and Serial which can be used to access many devices and services that do not have a separate binding.
For example, in a few lines of config I was able to connect my home automation with the Nightscout server I’m running. There is no Nightscout binding, I’m just using the HTTP binding. So would Nightscout be one of the services listed in this database?
I really appreciate that you are putting effort in rather than just whining about it. I really really appreciate that you are including our community in your effort. I know we tend to be a bit insular here which is probably to our detriment. I remember coming across a comment on a Reddit thread that said “today I learned people are still using openHAB.” This forum is the center of our community so people outside of this forum do not realize that not only is OH still around but it is rapidly growing.
But I’m not overly optimistic that the effort required to populate such a website will ever be enough to catchup let along keep up with the many new devices being added to both platforms every day.
But, to get started, there is the zwave database which our zwave binding currently uses. Though if you have a zwave device not yet in the database, we can get it added and supported in a couple of days. So ALL zwave devices can be supported now or within a couple of days.
The way we handle this particular problem is it is a requirement for the binding authors to provide a list of the devices that are known to work with their binding, if the binding doesn’t support all the devices for that technology. So you or someone can browse through the add-on readmes and if it has a list of devices known to work add those to the website and if not, add ALL the devices that use that technology. How to get the list of all the devices is an exercise left to the student.
I wish you the best of luck! And if you know of other areas where our two communities can collaborate or synergies I’m certain there are plenty here who would be happy to work with Home Assistant to make it happen.