Mine was a pizza delivery game called PizzaRush. I did finish it after about eight months of work, even sold a few copies. I was learning C, Mac programming, and a sprite toolkit all while making the game, so I can't believe it was ever finished.
One thing I should have learned was keep it simple. I was more or less forced into that because of my limited knowledge at the time, and as a result the game got completed. It seems too often I get grand ideas that are just too much and I end up not completing anything.
I've actually thought about revisiting the the game again. It was written seventeen years ago and was Mac-only, meaning it won't run today unless you're using a really old computer. Nowadays it would get the level editor and other goodies that were rumored to be coming in the third version (I just remembered...I actually completed a 2.0 version of it!)