LaTeX equation editors:

EqualX is probably the best options as it has already been mentioned. But unfortunately there are no binaries available for mac. There is also LaTeXiT which is only for mac! KLatexFormula Is the only cross platform option I could find so far:

LatexEqEdit (only windows), Laeqed (Java) and Ekee (for Debian Linux distros) are some of the alternatives you can try.

WYSIWYG editors or live preview:

There are also WYSIWYG LaTeX editors or editors with live preview. LyX is probably the best option as mentioned by others. But there are also Gummi and preview-latex plugin for Emacs:

If you want to have a full fledge text/word editor I suggest you to try TexMaths plugin for LibreOffice. It does not provide you with live preview though. Qilin editor is also a very interesting project. Although it is not a LaTeX editor it gives live preview of KaTeX and AsciiMath:

Mathematical software with LaTeX export:

TeXmacs is also another WYSIWYG giving you LaTeX output. It is a unique piece of software in it's own way. It has many other features too, like integration with many mathematical packages/languages such as Maxima, Octave, GNUPlot, Python...

I personally use WxMaxima though. It is very neat and lightweight. Consider it as an open source alternative to Mathematica. You do your symbolic/numerical calculations and then can export the equations easily by right clicking on them.

Jupyter Live Markdown Preview

There is an awesome new extension for Jupyter where you get live Markdown preview like the one we get on StackExchange forums: