Grow your own Telvanni Towers
One of the first mod's I ever released for The Elder Scrolls 3: Morrowind, It allowed the player to grow massive Telvanni Towers anywhere in the world. At the time I knew no programming languages and taught myself how to write scripts in the Morrowind scripting language (which is similar to Basic mixed with C).
After working on Morrowind scripts I found that I really enjoyed programming. When the Half Life 2 SDK was released I downloaded it on day one and got to work learning C++ using the Half Life 2 code base.
Having worked on Half life 2 mods for about a year I found that Kreedz Climbing was looking for a programmer. They had a few artists and level designers but had never found a programmer who could commit to the project.
The artists and level designers had been working on content for over a year when I got on board and within a month I had got the first beta release ready for shipping.