If you've ever needed to add complex equations to a PowerPoint or Keynote presentation or a Word document, then you need to know about LaTeXiT, an OS X application that typesets LaTeX without the need to create a file and run it through LaTeX. The images it creates are draggable to other applications. Of course, you have to know LaTeX to set up the equations, but if you need to typeset math, there's no way around that in any event.