The United States Internal Revenue Service lists about twenty sites that offer free E-Filing. Some also include free tax preparation. Each site has it's own qualifications, but most allow a maximum income of $54,000 and also have age range restrictions.
An easy way to find the ones you qualify for is to use the "Guide me to a Company" button. You fill out a form with your age, income, etc. and you get a list of the free tax filing sites that you qualify for.
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Some states also have free e-filing. To find out about them you should visit the website of your state tax bureau.