Developed my own programming language to teach programming at community college.
I needed an easy to learn language with as few brackets as possible cz these caused the most problems for beginners. Called it robocode. =)
Then i built an IDE around it where you have to program a little sheep to eat all gras in an area. The goal was to teach how to learn the syntax, the libary, debugging and to "see" the code run while the program and the little sheep runs, ..halt the programm, inspect variables, check the positions on the grass, ...i think you get the picture.
Later i built another IDE where you can program a Tetris.

robocode now also powers the calculation in our buisness application.

...i think thats my most successful project so far.

here's a screenshot of the RoboSheep IDE (be nice, it's a few years old) and the links to the download sites. I'm sorry, it's all german cz i never localized it.