An extension to the SPARC architecture. VIS stands for Visual Instruction Set, and is similiar to things like MMX and 3DNow!. It essentially adds fixed-width hardware SIMD extensions, useful for things like MPEG decoding. Sun's marketing page for VIS mentions the following applications:
How many of these are really relevant depends on how skeptical you are of the marketing department. For example, cryptography and DSP applications seem to cover most of the places in networking and telecom that SIMD could be used in. Whatever.
One mildly interesting thing about VIS is that Sun has written a library called mediaLib which takes care of common graphics operations for you, using VIS if it is available on the system (or normal SISD techniques if it is not).
VIS was added to SPARC in the SPARCv9 (aka UltraSPARC) revision of the architecture. Thus it is not available on any 32-bit SPARC system. However, VIS can be used in 32 bit applications running on UltraSPARC systems. A few additional instructions (proprietary to Sun's SPARC implementations) were added in the UltraSPARC III.
You can find out more about VIS by reading http://www.sun.com/processors/manuals/805-1394.pdf, which also has a nice high-level overview of the UltraSPARC I/II design.
Sun's list of VIS application areas was found at http://www.sun.com/processors/vis/