Notepad++ is a free (as in "free speech" and also as in "free beer") source code editor and Notepad replacement that supports several languages. Running in the MS Windows environment, its use is governed by GPL License. (Taken from official pages)

Notepad++ provides several advantages for developers. It's no huge Integrated Development Environment, it's just simply a light-weight text editor.

It provides

• syntax highlighting for various programming languages
• strong Search/Replace feature using Regular expression and being able to process open files or given folders
• line numbers
• auto-completion and parameters suggestion
• multi-editing
• macro recording and playing
• different style themes
• multiple execute commands
• many plugins to enhance the possibilities even further

All these features convince various developers to use this editor.