Not technically difficult, but I remember that it made me feel very proud: Back in high school (2004-ish) I made a small PHP script to generate a .m3u of my whole MP3 collection and I exposed this from my home PC. Since I could install Winamp on the school PCs this allowed me to stream my music collection.

Another one was a password rotation script for my university. There was an idiotic password policy that required you to change your password every 6 months via a web form, and if you tried to change it to a password you had used before you would get an error saying that the new password couldn't match the last 24 passwords you had used. This information was very useful as it allowed me to write a script that simply changed my password 24 times and then changed it back to the one I wanted.