As soon as I read the intro to Dr. Jerz's Modding, Machinima, and Motion Capture, I immediately thought of The Sims, but not for the reasons Dr. Jerz included in his introduction. The Sims really was my introduction into Modding. Users have been creating custom content ever since the release of the original game. The Sims Resource (TSR) was just one of dozens of sites I used to visit back in the old days of The Sims 1. Users created custom sim heads, clothing, furniture, carpet, wallpaper--you name it, and there was a mod.
I used to spend hours upon hours pouring over user-created content, downloading from various websites. This was how I discovered what a Trojan Horse virus was...one of the objects I downloaded completely crashed our computer (or something like that--I don't really remember), but after that I was pretty much banned from downloading stuff off of the internet for the Sims. Looking back, it definitely taught me a lot about Modding before I even knew what Modding was. In a way, it was an educational tool, because I needed to learn how to use zip drives and also use a special program to download the objects and clothing into my game.
One particular mod allowed players to turn their children sims into "teen" sims, which were really just children sims with stretched out arms and legs. They looked really freaky, but could do everything an adult sim could, which included "whoohoo."
Unfortunately, the mod also made the game unstable (for obvious reasons). However, the original modding of the first Sims games really opened doors for future Sims games. The Sims 2 and 3 both offer exclusive exchanges for user-created content, but sites like TSR still exist and thrive.