I'm looking for a Windows based visual equation editor which produces LaTeX code.
I need the functionality like in those online equation editors such as the LaTeX Equation Editor site
It should be an open source/freeware software (Portable installation preferred).
Best Answer
Have you tried LyX?
See also this list.
Any reasonably developed LaTeX IDE will have buttons for adding a variety of symbols as in the online one you linked to. For example, Kile and TeXmaker both have this capacity.
TeXworks doesn't yet seem to have this capacity (at least, the version that comes bundled with Ubuntu's TeXlive 2009 doesn't) but I expect it will soon...