Intel NUCs or similar clones Lenovo ThinkCenter Tiny or HP ProDesk Mini are good candidates. They are not worth their retail price but are often sold very cheap

Intel NUCs or similar clones Lenovo ThinkCenter Tiny or HP ProDesk Mini are good candidates. They are not worth their retail price but are often sold very cheaply on second-hand markets like eBay. I have one. Very quiet things and can handle a dense amount of workloads. Memory, storage and mini PCI devices are all upgradeable. These things consume around less than 8W at idle, so obviously they are not as low power as the RPi. But if you need to handle a lot of beefy services on the same machine then the power-performance ratio of these machines may be more attractive.