A variety of SPARCstations exist, including some reasonably high end models like the
SPARCstation 10 and
SPARCstation 20, which support
SMP and other cool stuff. They are all based on variations of the 32-bit
SPARC CPU. While both have been out of production for years, they still fetch a decent price on
eBay, high enough that I haven't bought one yet.
The SPARCstation 4 and SPARCstation 5 are quite similiar. The SS4 is basically a stripped down version of the 5, with less ability for expansion, but also significantly cheaper. Curiously both the SS4 and the SS5 came out after the SS20. Either makes a good workstation or small server. The SPARCstation 1 and SPARCstation 2 are old, slow, and cheap. At this point, I'd be willing to say that they're too slow to be worth buying. If you can pick one up for free, go for it, but really, they're slow enough (and heavy enough) that they're not even worth the shipping anymore.
The SPARCstation line replaced Sun's old line of 68k-based workstations, and have since been replaced themselves, by Sun's expensive and somewhat faster 64-bit line of UltraSPARC based systems (for fast, expensive, and 64-bit, see DEC Alpha). The line ranges from the Ultra 5, a commonly used workstation based on commodity technology like IDE and PCI, up to the exceedingly expensive Ultra 60 (with the Ultra 10 and some others in between).
Update (12-12-2002): conform reminded me that Sun's Ultra line is now also defunct, and has been replaced by the so-called Blade workstations. Last I looked, there were only three models available, implying Sun was looking to consolidate their product lineup a bit.