The "Morning-after pill" delivers a higher dosage of the progestin found in ordinary birth control pills and works by preventing ovulation. Known as Plan B and available over the counter for those over 18, if taken within 72 hours of unprotected sex, it is up to 89% effective at preventing pregnancy and most effective if taken within 24 hours.
Almost all oral contraceptives can be used as emergency contraception but specific dosing for each depends on the particular formulation. Ask your doctor or pharmacist about taking the appropriate dose. Side effects might include a change in your period, nausea, breast tenderness and abdominal pain.
In addition, having a copper IUD inserted within 5 days of unprotected sex is 99% effective at preventing pregnancy. Only the copper form, not the hormonal version, has been studied as emergency contraception.