I just installed iTeXMac2 (available at
http://itexmac.sourceforge.net/iTeXMac2.html) on 10.5.1. It's running fine so far, although I haven't used any of the bibliography features yet.
I installed that after reading a helpful list of LaTeX options for Mac at
http://guides.macrumors.com/InstallingLaTeX_on_aMac
To be clear, installing this takes a few steps:
First, you have to install an application called i-Installer by someone named G Wierda. This is done in order to, second, install gwTex, which is the actual TeX engine that does the typesetting. You can install this by running iInstaller and choosing gwTex from a list of installable software. Finally, you can install iTeXMac2 which is a GUI and manager, using the file from its sourceforge homepage.
As noted on the site listed above, there are other packages available, with simpler installation options. My friend who is a first-time LaTeX user has tried TeXShop and is satisfied with it.
Good luck with whatever you're writing in 10.5!