A piece of mathematical software, originally for the PowerMac, now developed by a company called Pacific Tech (http://www.pacifict.com). It has a somewhat unusual history of how it was created - see http://www.pacifict.com/Story/ for details, or go to http://www.thisamericanlife.org/ and search the archives for "Should I Stay Or Should I Go". Basically, the developer kept working on it at Apple, after having been fired by Apple, but that summary doesn't do it justice. There ought to be lessons in it.

A new one is shipping in MacOsx 10.4 "Tiger", and it should be availabe for both Mac and Windows, according to the website.