Here’s a new species that I haven’t seen before, the rack-mount clusterboard using SBCs:
A home-grown 48-board setup from the folks at PINE64. In this case, the RockPro64 SBC includes a Gigabit Ethernet port and a PCIe 4x slot, which could make for a ton of mix-and-match with storage and other gear with open-ended expansion (start small and cheap, then just add more nodes as necessary). Combined with inexpensive 1 Gbit and 10 Gbit switches (e.g. an 8-port MikroTik at $269), things could get interesting real fast!