I didn’t really have any concept of importing libraries or using open source software, so literally every part of this huge and complex application was custom-written by me. All UI elements were custom designed from shape primitives, including a full time series graphing utility and a drag-and-drop programming language with compiler. The database was built by me using a custom binary data format I had invented and lots of disk IO.
It was extremely fragile, but it was also the most beautiful and functional app I have ever used to date. I wish it weren’t a Java applet so I could host it on the web again, but alas.