My check engine light is on. I have check the fluids, gas cap, and visible belts. What else might cause the check engine light to be on and remain on while the car's power is on?

 

Where to begin......
any one of 100+ sensors, EVAP system leak, blown fuse, shorted wire...

Pull your codes, go from there.
People on here say you can pull the codes by cycling the key, but I didn't know that could be done since the 80's....

 

Lol.
It all depends on your vehicle, I guess.
Usually, you can follow one of the following two ways to pull a code:
1) Mine is a '96, and it still does it the old-school way. HERE is the old-school way.
2) The newer way is to cycle the key the same way as told old way (only, instead of 3 cycles, it might take 4 or 5) A code might be displayed on the odometer display (if you have a digital odometer). This usually works on vehicles made after '96 or so. The codes are defined here.

Another way, if your vehicle is about a 1996 or newer, you can take it to virtually any autoparts store, and they can read your codes. If they try to charge you, take it somewhere else, as most autoparts store read codes for free (which is why you never take it to a dealer just to get a check engine light diagnosed!)
Please, please, please, when posting back, post the actual code. It will do us virutally no good to say "it said something is shorted" or "there is a leak somewhere" or "a sensor is bad," because, like dodgedlr said, it could be one of a million things.