Websites offering free printable worksheets that teachers can assign as homework include GreatSchools, Tlsbooks and Education.com. The worksheets are most often available for elementary school students, and they're not suitable for commercial use.
GreatSchools offers worksheets for students in preschool through fifth grade as well as activities for students through middle school. Worksheets are available to practice writing, reading, spelling, math and science, and some are geared toward the Common Core Standards.
Tlsbooks offers preschool through sixth grade worksheets as well as multi-grade ones in areas including the language arts, religion, music, math, history, writing, foreign languages and geography.
Education.com offers elementary, middle and high school worksheets and printables relating to foreign languages, art, math, reading, social studies, writing and science.