WYSIWYG LaTeX editor for maths

As already mentioned in my comment above, EqualX is a nice equation editor available for Linux, Windows, and Mac. It requires a working TeX Live or MiKTeX installation available.

The generated images can be directly dragged and dropped into other applications. The properties bar (activate with Window > Properties Bar) between image and source area allows to change color, transparency, and size of the generated images.

This is one of online WYSIWYG math editors. You can easy use it to type mathematics without knowing latex but it also provides import/export Latex. Input is faster because it uses Suggestion Box, Character Recognition,... This also support drawing Diagram (Input math in Diagram)

Mathcha Editor

LyX has a relatively decent math editor. By selecting View->View Source, one can directly see the resulting LaTeX code and easily copy&paste it from there. Furthermore, it is easy to typeset the result via LaTeX to get a quick impression on the actual look of the result. The immediate representation of the resulting LaTeX source helps me a lot to actually learn the respective LaTeX commands.

Within the math editor, one can either choose symbols and environments from the toolbars or, if known, just type the respective LaTeX / AMS math commands directly, which LyX then converts on the fly to its WYSIWYM representation. Furthermore, it is possible to define own math macros. This provides for rapid entering of formulas.