ASRock has also pushed into some of Supermicro's traditional product segments via the "ASRock Rack" brand. I have no idea how big that business is, though.
Gigabyte and Zotac also comes to mind, resemble Asrock.
There's a bunch of other kit too, but https://servethehome.com reviews a bunch of the various rack systems.
Example of some late January posts. Albeit none are of the "fits lots of GPU" sort that is helping propel Supermicro, but these folks all have those offerings too,
