Notepad++
I use Notepad++ to code this site.
Notepad++ by Don Ho allows be to view code with syntax highlighing in a theme of my choosing. I like a dark background to code with. The syntax highlighting helps me catch any typos. I use Notepad++ for HTML, CSS, PHP and JavaScript, but there are plenty more languages that it recognizes.
I used to use a program called EditPlus, which is not free. I then discovered the free Notepad++ and it wasn't lacking in any of the features I needed.