What happens if you forget to file taxes?

Allright, I'm guilty of this. I filed an extension in April and pre-paid the amount that I thought would be due. After that we moved and got really busy and I totally forgot about the Oct. 15 deadline to file my taxes. Now I'm afraid to even look at how I'm going to be penalized for this. Anyone know what the penalties are? I think I've already paid all the taxes I owe, but haven't filed a return.