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.