During Microsoft's development of Project Longhorn (later named Window's Vista) I was retained as a remote contractor on the accessibility team to re-write "Narrator", Microsoft Window's built-in screenreader.
The Narrator work was a mix of C++ to build two unmanaged DLLs that can run at login and hook into system events and C# to handle the UI. A number of innovative features were built but then later removed to simplify the product.
This code has endured seemingly unchanged through Windows 7 and 8. Sometimes it is strange to think code I wrote sits on hundreds of millions of harddrives around the world.