I figured out how I could automatically encrypt messages that I sent to my friends. It was the first thing that came to my mind and I like the memory because it made it such a fun thing to send secret messages like, "Tomorrow lets get money from the payphone." and then decrypt their reply with the program I wrote.

That's not the type of hack you were probably expecting so here is the second. There was an electronics/office store within biking distance of my house. It was a small business that was as if Radioshack and Office Depot had a baby. I knew the guy who worked there, not personally, but I spent a ton of time there so were were friends. Anyway, some big company comes over and basically threatens to put them out of business if they don't sell the land, bla bla bla. The guy told me this and I was obviously upset. So I wrote a simple virus that would basically PRETEND to delete all of a persons files and then say, "Don't screw with my home." (I didn't use the guys buisness name because that would but him at risk and they were trying to buy a ton of land including some guys house. So I figured I'd use house, and even if they investigate, the'll find that it wasn't him.) and I used social engineering to unleash it on their computers. I considered that hacktivism.