Notepad++ is a free source code editor and Notepad replacement that supports several languages. Running in the MS Windows environment, its use is governed by GPL License.

Quoting from the Notepad++ home page:

Based on the powerful editing component Scintilla, Notepad++ is written in C++ and uses pure Win32 API and STL.

Notepad++ is licensed under the GNU General Public License.

Notepad++ on Wikipedia