Notepad ++ is an evolution of the classic windows notepad, with which you can write documents in txt, ie notes. It is a very light software and can also be used to view and write source codes. It recognizes a vast amount of file formats. If you want to write only source code I recommend sublimetext, but notepad should not be missing on your computer because it can always be useful.
Sublime Text is a sophisticated text editor for code, html and prose – any kind of text file. You’ll love the slick user interface and extraordinary features. Sublime Text may be downloaded and evaluated for free without an expiration time limit, however a license must be purchased for continued use.
As a better alternative to Sublimetext, ie as a pure sourcecode editor, brackets is the best software. It’s free (opensource) and allows you to view the live preview. Because of the consistent graphics it can run a little slow.
Geany is a small and lightweight Integrated Development Environment. It was developed to provide a small and fast IDE, which has only a few dependencies from other packages. Another goal was to be as independent as possible from a special Desktop Environment like KDE or GNOME – Geany only requires the GTK2 runtime libraries.