U.S. Citizenship and Immigration Services - Lots of information and forms.
U.S. Citizenship and Immigration Services: Study Materials for the Naturalization Test - Free materials to prepare for the Civics test as well as the English test.
Learn About the United States: Quick Civics Lessons for the Naturalization Test - 36 page brochure from theOffice of Citizenship explaining the answers to typical questions from the naturalization test.
U.S. Citizenship Exam Preparation - Timberland Regional Library system subcribes to the online Learning Express Library, which has exam preparation in English and in Spanish. You will need a Timberland card to access this from home.