LaTeXiT is a little tool that aims to help you quickly typeset LaTeX equations.
LaTeXiT doesn't require you to spend your time with file creation, preambles, or other related actions. The PDF image obtained after processing the equations can be exported by drag-and-drop to any application that supports such files.
This is very useful to insert equations in presentations made with Keynote or Powerpoint. LaTeXiT also features an application service, so that you can type and transform equations directly in most text editors (Pages, Nisus Writer Express, TextEdit, etc).