The TeX collection DVD includes MacTeX, which is a Mac-tailored version of texlive; details may be found on the TUG web site. If you don’t have the disc, you can download the distribution from CTAN (but note that it’s pretty big). MacTeX is an instance of texlive, and has a Mac-tailored graphical texlive manager, so that you can keep your distribution up-to-date.
Note that installing MacTeX requires root privilege. This is a pity, since it offers several extras that aren’t available via a standard texlive, which aren’t therefore available to “ordinary folk” at work. For those who don’t have root privilege, the option is to install using the texlive tlinstall utility.
OzTeX, by Andrew Trevorrow, is a shareware version of TeX for the Macintosh. A DVI previewer and PostScript driver are also included. OzTeX is a Carbon app, so will run under Mac OS/X (see http://www.trevorrow.com/oztex/ozosx.html for details), but it is not a current version: it doesn’t even offer PDFTeX. A mailing list is provided by TUG: sign up via http://tug.org/mailman/listinfo/oztex
Another partly shareware program is CMacTeX, put together by Tom Kiffe. CMacTeX is much closer than OzTeX to the Unix TeX model of things (it uses dvips, for instance). CMacTeX runs natively under Mac OS/X; it includes a port of a version of Omega.
Further information may be available in the MacTeX wiki. The MacTeX-on-OS X mailing list is another useful resource for users; subscribe via the list home page