On Mac OS X, possibly a typesetter's dream OS due to Quartz and the Unix base, I use TeXShop, iMacTeX, and BibDesk with the TeXLive-teTeX distribution.
TeXShop and iMacTex are both TeX IDEs (with a TeX aware editor, front end to teTeX, and hooks into DisplayPDF for viewing typeset documents), with TeXShop being further along for my uses, but iMacTeX supports more encodings beyond TeXShop's ISO Latin and Mac OS Roman. iMacTeX is developed by Jerome Laurens, who was a contributor to the TeXShop effort, but decided to split off and pursue a different development path.
TeXShop, by Richard Koch and Dirk Olmens, is my general TeXery workhorse application on OS X, and it's great. There's not much more to say. Download it and hack TeX right now!.
Why use two different TeX front ends on the same platform? They're both free and good! Why not?
BibDesk is a nifty tool from Michael McCracken that allows you to organize and use BibTeX bibliographic databases, and comes in very handy when you want to search a ".bib" database and cite papers.
TeXLive-teTeX, by Gerben Wierda, combines the TeX program base of TeX Live (the central TeX development system for Unix) with the package and macro support of teTeX, one of the best TeX distributions around. What's more ubercool are the neato OS X installers Gerben has built.
You might be wondering what compiler I use for all this. On OS X, there's scant better choice than PDFTeX.