Emacs running under X will do basic formatting (italic, bold, larger text, etc.) all on its own. There are packages out there that will do more elaborate previews. Googling gives: http://savannah.gnu.org/projects/auctex

As scott_s suggested, LyX is a prominent LaTeX WYSIWYG editor. I use TeXmacs (http://texmacs.org) on Linux because I input a lot of Greek characters and do a lot of Maxima computations while taking class notes, and the ease of Ctrl-Enter makes it like Mathematica notebooks but much faster for writing. It doesn't save natively in LaTeX format like LyX does, and doesn't implement all of LaTeX, but it's an option worth considering if you're in a similar situation.

As has already been mentioned, LyX and TeXmacs are possibilities. Try plain LateX a few times before jumping for the wysiwyg option. It may be nice to see the final output immediately but the benefit of macros combined with the advanced capabilities of editors like emacs makes pure latex a breeze to work with once you get the hang of it.