The mother of all dictionaries, the Oxford, is now available for Macs (PPC and Intel) on CD with half a million words, and the ability to trace word usage through more than 2 million quotations. This version 4 edition CD has the full text of the OED 2nd edition, plus supplementary volumes, full text search, options to customize the entry display, and a variety of ways to display the results.
The Dictionary is designed to be copied to your hard drive, and requires at least a G4 processor with 867MHz or greater or an Intel Core Duo 2.13GHz or faster. The dictionary can run on either OS X 10.4x for 10.5x.
If memory serves the OED hasn't been on the Mac since version 1, and this is a welcome return. The CD is pricey, US$295.00, but buying the printed version is more than $900.00 and takes 20 volumes. Amazon has the CD version for $212.40.
You're probably saying "hold it -- my Mac has the Oxford Dictionary built in!" You'd be right, but it is a cut down version, with about 2/3 of the definitions missing, and for U.S. users, we get the Oxford American Dictionary, not the English Dictionary.
The CD release is not perfect. It has no way to save searches or info out for later research, and the dictionary does not integrate into the existing dictionary on Leopard, so the two don't talk to each other. The GUI is ugly and not Mac like. If you are looking for the last word in dictionaries, however, this is it.