How often do you pay taxes?

I, being in the United States, always thought that you pay taxes once a year on April 15. Until I was slapped with an "underpayment penalty". Turns out that I was supposed to do "estimated taxes" and pay them four times a year. Because freelance income was not taxed "out of the paycheck".

For you freelancers, do you pay taxes just once a year? Or pay four times a year? Also help if you give context to your answer by telling us what country you are referring to.