NP++ is great but sometimes you want something even more minimal... I for example hate NP++'s icon toolbar and it's cluttered menu bar and would never use it even though I acknowledge that it's a great tool (just not for me).
Classic notepad.exe with basic syntax highlighting would be amazing...
Which means you have to check each and every formula and we all now how difficult it is to read and understand excel formulas we didn't write ourselves....