Check Engine Light
Here's a bit of knowledge to add to the internet's infinite repository.
I have a 2005 Chevrolet Cavalier (base model), and just recently I drove home to California and then back to school. A couple days later I noticed that my check engine light was on. So naturally I was worried and took the car to Pep Boys to have it checked out (since they do the examination for free).
Result: My gas cap was loose. It's the type that you press and twist once to take off and then you twist it once to put it back on. Apparently I hadn't twisted enough (it makes a horrible clicking noise when I keep twisting).
Lesson: If your check engine light comes on, check the gas cap first.
Happy driving!