This is a free source code editor which supports several programming languages running under the Microsoft Windows environment. Notepad++ can be used as a great freeware replacement for Notepad.
This project, based on Scintilla edit component (a very powerful editor component) and written in C++ with pure win32 API (i.e. without MFC, that ensures the higher execution speed and smaller size of the program), and is also under the GPL Licence.
This project is mature. However, it's a one-man-project, so there're still some bugs and missing features.