I've seen people mention Maya and Maxwell Render, what else is there?
After Effects, other 3d apps (Cinema 4d, Blender), some video compressors (HandBrake). Now that I think about it, there aren't many in the design field that really take advantage.
I'm sure many scientific and math programs are multi-core.
When I run Compressor it doesn't appear to be taking advantage of all 8 cores.
Use Apple QMaster (comes with FC Stuido) to set up multi-core rendering on a Mac Pro:
http://www.larryjordan.biz/articles/lj_compressor_multi-processor.html