A model of computer, made by Sun Microsystems, based on the 32-bit RISC architecture known as SPARC. It is, naturally, part of the SPARCstation line of computers, which actually vary significantly in features and power.
The SPARCstation 2 (often called the SS2) is powered by a 40 Mhz TI-made chip, and supports up to 64 megabytes of memory (128 with an additional add-in chip, but it's very hard to find nowadays). It has built in Ethernet (AUI), and can be converted to use RJ-45 with the appropriate widget. It also has built-in sound and a SCSI controller. It can be upgraded to use an 80 Mhz CPU with a third-party upgrade. In that case, it will be about as fast as an 80-100 Mhz 486. One sucky thing about the SS2 is that it is based on the SPARCv7 architecture, which is so RISCy that it doesn't even have a multiply instruction! So it can be really slow if you're doing certain things. Fortunately, there aren't that many applications one would wish to run on the SS2 that are affected by this (mostly crypto, graphics, numerical stuff, etc).
It has three SBUS ports for add-in graphics, additional ethernet or SCSI ports, or whatever else you might want. SBUS is a bus system used only on Sun machines, and it is actually still used today on some Sun boxen, including the mighty E10K (aka Starfire). There is even a SBUS/PCMCIA adaptor called Nell, I hear.
While the SPARCstation 2 is quite old, it is still useable for light DNS, web, or mail serving, or as an experimental box. One thing really going for it is that it is very easy to netboot completely diskless, because RARP and TFTP is built into the BIOS (which called an OpenFirmware on a SPARC). So you can use it to experiment with a new operating system (in my case, NetBSD) and architecture. This is also recommended because most old SCSI drives are rather louder than most people would like, especially if they're keeping it in their bedroom.
Operating systems that can run on this machine include Solaris (it will be slow), SunOS, Linux (also slow; the SS2 MMU is poorly supported by Linux), NetBSD, and OpenBSD.