https://osgameclones.com/ is the best aggregate for this kind of thing.
https://libregamewiki.org/ also looks like something relevant.

Maybe now that we have a messy thread of various links, we can go check whether some of them are not yet listed in osgameclones.

All right, good stuff.
But I’m slowly getting a feeling that there are quite a few “European alternatives” websites right now.
Will make a list of them once I have time.