SPARC is Sun Microsystems' "S"calable "P"rocessing "Arc"hitecture (at least that's one version, might be "Processor"). This is the most recent generation of Sun's famous line of workstations, also known as "sun4". It goes back quite a ways, already, to the early 90s. Debian supports the more civilized models -- some of the early ones are unsupported, or poorly supported.
There are a bunch. They can generally be divided into 32-bit (old) and 64-bit (new). 32-bit has been broken in 2.4 due to lack of hackers on old machines.
The earliest supported 32-bit architecture, but some early exotic sun4c machiens might not work -- this is primarily a kernel issue. Includes ["SPARCstation"] 1 (aka ss1), 2, IPC, IPX, ELC. Read the Sun Hardware Faq at www.sunhelp.org for more information on these and older machines. Linux doesn't run as fast as ["NetBSD"] on these machines -- Linux doesn't use the hardware context storage correctly (there was a recent DaveM post to linux-kernel about this), and he doesn't feel like fixing it. Someone else may be working on it though.
A much more civilized class of 32-bit ["SPARCs"]. Include ss4/5, LC/Classic. These chips are all on uniprocessor workstations, and go up to 200 ["MHz"] (on the best ["TurboSPARC"]-boosted ss5's, IIRC). Equivalent performance to a Pentium Classic, but with better FP. Tadpole ["SPARCbook"] 3 laptops are in this class. Very well supported.
The "funky" class of 32-bit ["SPARCs"], the multiprocessor machines. ss10, 20. These can support up to 4 sun4d chips, which go up to ["150MHz"] or faster (each). Your PROM revision limits what the machine supports.
UltraSPARC (I, II, IIi, III)
64-bit ["SPARCs"]. ["SPARCstation"] Ultra 1,2,5,10,30,60,80, and Enterprise 250/450/1000/etc. Up to 32 or 64-way SMP on these machines, but the processors themselves are (at the moment, anyhow) slower than the fastest P4s or Athlons (though they may be a hair faster clock-per-clock, they only go up to ["800MHz"]). But you buy these machines for the seamless scalability to 32-way. These are very well supported, though individual framebuffers or other peripherals may not be.
SPARC is one of the most mature Debian ports. Ben Collins (past Debian Project Leader) is the lead maintainer. The list is firstname.lastname@example.org, and there appear to be quite a few people running Debian on SPARC (certainly more than 2-3 years ago).
SPARC is one of the more unusual architectures. It's RISC, but instead of just having "a lot of registers" like Alpha or MIPS, it has a unique circularly-linked list of 128 registers, of which you can see a 16-register "window" at any one time. It's "circularly-linked" because you can walk the window in either direction. In general the architecture is extremely fast for floating-point operations, but suffers on any computation that relies on the stack overmuch (memory bandwidth has historically not been fantastic, which Sun has remedied by advanced cache architecture). The machines themselves are extremely well engineered and specified -- all the Sun-specific internals are IEEE standards, including the SPARC ISA, Sbus (the bus in older ["SPARCs"]; newer use PCI), etc etc.