While "beige box" is a generic term referring to the ubiquitous beige computers of the 90s and 00s, in this context it refers to the old computer I picked up for $20 in a parking lot sale my local e-waste recycling and community tech education center was holding. It came with a 400Mhz AMD K6-2 processor, 384MB of RAM, a 3.25" floppy drive, a CD-RW drive, a Trident 9750 AGP video card, a 2 port USB 2.0 PCI card, and a US Robotics Sportster 56K modem.
I've added a SoundBlaster PCI512 sound card, a 3Com 3C509B ISA network interface card, and an SD card to 40 pin IDE adapter to let me use SD cards as hard drives. I also designed an 3.25" floppy bay adapter for panel-mount USB ports to allow breaking out the motherboard's two built-in USB 1.0 headers.
I've successfully run Windows 98, Windows 3.11, OpenBSD, NetBSD, Tiny Core Linux, and 9front so far on it. OpenBSD removed Trident support from their X fork, Xenocara, years ago, so drawing to the screen is very laggy. NetBSD does not provide a binary for the Xorg Trident driver, but it seems like it should be possible to build it from the source tree by bootstrapping "modular X." Unfortunately trying to build it on the beige box itself resulted in a compilation process that took a week and ended in an error that was lost in the scrollback. Any further attempts, it seems, will require cross-compilation.
9front has been suitably resource-light. Despite apparently having no support for my video card, the GUI feels more responsive than in, e.g., OpenBSD or NetBSD. It looks like it achieves this by dropping frames, which I frankly much prefer to having a lag between dragging a window and seeing the arc of its motion.
I'll be replacing the PCI sound card with an ISA SoundBlaster 16 soon, as the VIA chipset on the motherboard requires drivers (apparently due to a slightly incorrect PCI implementation) that aren't available for DOS, Windows 3.11, or 9front (at the very least). Without those drivers, the PCI bus doesn't work.