The best IDE for web developers! That is the first thing that comes to my mind every time someone asks me “What is notepad++”.
I really love this application, It supports many languages such as php, ruby, html, etc, For those whose using frame works and needs specific extensions like in cakephp they use .ctp instead of html. Notepad++ made it possible to adapt in different file extensions all you have to do is to edit the style configuration of your IDE and add the ctp format under HTML style.
The file size is small and its free and they also have versions for different operating systems.