A great online editor is available at formulasheet.com. It provides a live preview of the formula that you are creating so gives direct visual feedback. It has buttons to press for symbols and constructs like fractions and matrices so you can create the LaTeX code without knowing all LaTeX commands. It runs out of the browser so can work on Windows machines.
It also has additional options such as font, text color, background color, render resolution, and allows you to copy-paste the formula that you created to MS Word 2007+ using MathML.