Where can I find free source code online to learn Unit testing on

I have started this course on Pluralsight about testing .Net application(desktop application, WinForms) with NUnit because I want to learn how to test code(I am a tester). The project I work on now has a huge application and the source code is too complicated for me right now. Where can I find the source code for a desktop application(open source I guess) so I can start learning how to unit test by referencing that code to my test project in Visual Studio. Thanks :)