Yet another of the Sun SPARCstation line of computers, based on a SPARC CPU. It comes in 70, 85, 110, and 170 Mhz versions. It uses SBUS, like the rest of the SPARCstation line, and can take three SBUS cards and up to 256 megabytes of RAM (it has 8 slots that can be filled with 8 or 32 megabyte sticks).
The SS5 uses SCA hard drives rather than the normal SCSI. SCA provides power and extra control information in the same set of pins that the data goes over (SCA uses more pins than SCSI-2). It can hold two SCA disks, one normal SCSI drive (usually used for a CD-ROM), and a floppy.
The usual OSes to run on such a beast are Solaris, SunOS, Linux, NetBSD, or OpenBSD. Linux and *BSD are not particularly stable on the 170 Mhz version, though. This is because while the first three (70, 85, and 110) are based on a TI-made SPARC chip, the 170 is based on a completely different chip, and even a different MMU, IIRC. This is a shame, since another 60 Mhz would really help performance. C'est la vie. Of course Solaris and SunOS don't have problems with any of them.
These machines are rock solid, and, while a bit slow compared to an Athlon or G4, still make an excellent small server.
It's roughly in the same class as an SGI Indy; an old Unix workstation that still runs fairly decently. However, the SS5 is a bit more oriented towards server stuff (the Indy includes all kinds of technology, like a webcam and ISDN support, that you would never need on a server).
As of August 2002, you can get a "base" SS5 (64 megs of memory, 2 gig disk, 110 Mhz CPU) for about $40 on eBay. I recommend going for as much memory as you can get, these older machines can really use it, and it doesn't cost much at all.