Lethal Injection is now the standard execution method. Since the death penalty was reinstated in 1976, only about 150 executions have been by electric chair, about 10 by gas chamber and less than 5 by hanging or firing squad.
The Lethal Injection is usually 3 different chemicals: one which numbs the person, one which paralyzes their muscles (including the lungs) and one which causes a heart attack.
How Lethal Injections are given is now under review by the U.S. Supreme Court. Questions have been raised about whether the current cocktail of drugs in the injections cause the prisoner to feel pain which he cannot express due to the paralyzing chemicals. If so, it might be judged to be in violation of the Constitution which prohibits "cruel or unusual punishment." The Supreme Court is expected to rule on this issue in the Spring of 2008.

States which still have Electrocution on the books:
AL, AR, FL, KY, NE, OK, SC, TN, VA

Most of these states, with the exception of Nebraska, also use Lethal Injection.
Arkansas allows those prisoners sentenced to death before 1983 to choose between lethal injection and electrocution.
Kentucky and Tennessee does the same for those sentenced before 1998.
Oklahoma would use electrocution only if lethal injections are found to be unconstitutional (and firing squad if electrocution is also found unconstitutional).

States which still have the Gas Chamber on the books:
AZ, CA, MO, WY

All of these states also use Lethal Injection.
Arizona allows prisoners sentenced to death before 1992 to choose between lethal injection and the gas chamber.
Wyoming would use the gas chamber only if lethal injections are found to be unconstitutional.

States which still have death by Hanging on the books:
DE, NH, WA

All of these states also use Lethal Injection.
Delaware allows prisoners sentenced to death before 1986 to choose between lethal injection and hanging.
New Hampshire would use hanging only if lethal injections were not possible.

States which still have death by Firing Squad on the books:
ID, OK, UT

All of these states also use Lethal Injection.
Oklahoma would use a firing squad only if lethal injections and electrocution are both found to be unconstitutional.
Utah would use a firing squad only if lethal injections were not possible.