You may recall that back at WWDC'05 when Jobs announced the switch to Intel, one of the companies he invited on stage to discuss creating Universal Binaries was Wolfram Research, whose flagship product Mathematica is probably the leading desktop mathematics application. Mathematica has now reached version 6 and the new features are manifold, representing (according to Wolfram) the "most important advance in the 20-year history of Mathematica." Not having much understanding of these things beyond attempting to use it back in college to do my calculus homework for me, it does seem that there's a lot new, including Dynamic Interactivity and "over 1000 new computational functions & interface enhancements."
One downside of the new release for Mac users, however, is that version 6 remains a 32 bit application in OS X despite the availability of 64 bit versions for Windows, Linux, and various Unix flavors. Mathematica 6 is available now at a variety of different price points (education, etc.), with the standard professional version coming in at a cool $2495 for Mac.
Update: It was pointed out in the comments below that, although it is not enabled by default, it is possible to get 64 bit support on Intel in OS X by following these instructions. Thanks, Geoff!