Equation Service
I'm surprised that I didn't mention
Equation Service here years ago; it's a sweet little service for Mac OS X that lets you write LaTeX equations just about anywhere (TextEdit, Keynote, etc.) and render them to images in place with a keyboard shortcut (or item in the Application->Services menu). This was one of my favorite utilities when I was in school, but I haven't set it back up on Leopard yet.