PDF Equation gives you quick access to LaTeX's typesetting power in almost every Mac OS X application. Just type a LaTeX snippet, render a it and drag the resulting PDF file into your Keynote presentation or wherever you need it. PDF Equation is document-based, so you can save your equations for later use.
Version 0.7 adds a color well and a customizable LaTeX preamble
Mac OS X 10.4 or later, LaTeX.
*Previously available here