MathType, a graphical equation editor for Mac and Windows that can output TeX and MathML. Commercial, $97; $57 for students.
LaTeX Equation Editor is a Mac utility that lets you type LaTeX equation mark-up in a box, then whips up a PDF file that can be dragged into Apple Keynote directly from the window. Seems like it could be convenient for other equation editing purposes, too. Free. See also EquationService, also free, requires that pdflatex already be installed.
MathMagic is another graphical equation editor that many writers and publishers are using. It gives you very beautiful equations in publishing quality and many control options for the shape of equation.
Free trials can be downloaded at www.mathmagic.com.