Oooohh! Notepad++! Sweet memories for me! :-) Ever since I became an Apple-adept I haven't used notepad++ but it was indeed quite nice (back in the days). SublimeText isn't truly OpenSource / Free but "EvaluationWare", I think? You could use it indefinitely but a "nagging screen" asking you to buy it keeps popping up once in a while.

Atom IO editor is built on top of "Electron", which is built on top of "Chromium" (which is the engine of webbrowser Chrome). Atom IO is very nice to work with, very user-friendly, and extendable. But, it's kind of slow (for bigger projects). Also built using "Electron" is Microsoft Visual Studio Code. That's free to use as well, yet is very fast (it has a somewhat different user interface).

So, although I'm not a Microsoft fan per se (I'm not at all actually :-) ), Microsoft Visual Studio Code is maybe their best product ever.