Sun Enterprise 420R
As mentioned elsewhere in the museum, I was nutty for symmetic multiprocessing before multi-core x86 made it ubiquitous. So when I found this fully loaded Enterprise 420R at auction, I knew I had to have it. All four UltraSPARC CPU slots and the full 4 GiB of RAM were populated. And as a rack-mount server, it was loud when powered.
It fit a bunch of hardware including PCI expansion cards into a long, 4U rack-mount chassis. Unfortunately I didn't take any photos of it so below you'll find someone else's.
The Linux kernel still supports the sparc64 architecture as of 2018.