There are two possibilities (well, actually just one…):

1. Gerben Wierda's i-installer: download the i-installer application and use it to install and keep up to date the package gwTeX based on TeX Live. Mind that Wierda dropped the support for its i-installer; therefore the future of this software is unclear. Moreover, the i-installer may conflict with any of your custom installed software, so: do not use by any means.
2. MacTeX packages: this is the new “standard” for LaTeX on MacOS, it is based on the official TUG TeX Live distribution. Just install the “TeXLive-2007.dmg” (full TeX Live distribution from TUG, unmodified).

You can use Aquamacs, an easy-to-use, Mac-style Emacs for Mac OS X (sic.). It comes with LaTeX support built in. The only caveat is that you need to install ispell (or aspell) to get the internal spell checker to work. To this purpose you can use MacPorts or cocoAspell.

There are other alternatives, like iTeXMac or TeXShop.

Apple Preview.app doesn't reload the file when it changes, which is king of annoying. A good previewer for latex is PDFView. You can use TeXniscope which works with DVIs as well; but it doesn't seem to play well with intel macs.

Check out LaTeXiT, handy in combination with Keynote, Powerpoint, or other software in which you want to include latex snippets.

• Getting Started With TeX