Microsoft has released the source code of its original Windows File Manager. The File Manager was a replacement for managing files through MS-DOS, and allowed Windows users to copy, move, delete, and search for files. It was the part of Windows 3.0 and is 28 years old. The move is more of an appeal to those nostalgic for the 90s than anything else, but the open source code does compile and run in Windows 10.
Checkout the source code on Github.