Back in 2012 I was learning Python and had a long plane flight, so I decided to try and write a Sudoku solver. I spent probably ~6 hours putting this together before landing, so it only solves using a small number of techniques and does not completely solve most puzzles.
I tried to finish it later that year, but the code is so confusing (Pat, I would like to buy a comment) that even I could not figure out what was going on. I would probably be better off starting from scratch if I were to revisit this.