A SparcStation 20. This is a SPARC system insize a pizza box chassis. The architecture code is sun4m, which, of course, is part of the sun4 series. They can have up to 512 MB of RAM, in 64 MB SIMMs of 200 pins each. They feature the Sun MBus processor bus, which lets you have up to 4 CPUs if you get the right MBus Modules (double-width). With normal (single-width) MBus Modules, you can have (as one might expect) 2 CPUs in it. They run Solaris, Linux, or almost any of the BSDs rather nicely.
Onboard hardware includes 10BaseT Ethernet, SCSI with two internal SCA connectors (special drive sleds required), serial ports, and other goodies you'd expect. Also, the chipset for the SX framebuffer is on the motherboard, although you'll need a VSIMM to use it - more on this later.
The machines also feature Sun's SBus, a peripheral bus whice lets you add stuff like 1-port or even 4-port ethernet cards, framebuffer cards, other SCSI adaptors, PCMCIA host adaptors, and more. Plenty of this stuff, including the machines themselves, can be found on eBay for reasonable prices.
As for the framebuffers go, I'd highly recommend getting your hands on a VSIMM, whether the 4mb or 8mb variant. The included SX (aka cgfourteen) chipset is pretty fast, and it's probably the cheapest option available for running 24bpp on the SS20. Cheaper display options include the LEGO ("Low End Graphics Option"), aka cgsix or GX/GX+/TurboGX/TurboGX+. They have a maximum of 8bpp and have various supported resolutions. More on this can be found if one goes to google and enters "sun framebuffer faq" and hits their lovely "I'm Feeling Lucky" button.
Sun's OpenBoot firmware is also pretty neat. Definitely blows away the PC BIOS. It is basically a Forth interpreter with a lot of special features for booting and testing the SPARCstation. You can boot off of disk, floppy, CD-ROM, or even Ethernet using RARP and TFTP.
If it's of any interest to anyone, the SS20 from which I'm noding this sports two SM71 75mhz MBus modules, 384mb of RAM, an 8mb VSIMM, and a TurboGX cgsix card, as well as one 1gb SCA drive with Debian, and a 2gb drive with Solaris. It works well and makes a good system to use.