Open Educational Resources (OER) are free on-line resources for student learning. Some are individual activities, others are complete courses and textbooks.
www.opened.com: A vast collection of online resources brought into one place.
CK-12: Complete textbooks with activities, videos and more that can be customized to what you want
EdReady: Students take a pre-test (in Mathematics--content goes down to 5th grade and into University Calculus, in English it's 11th and 12th grade content), and then lessons and assessments are customized to skill deficits. Can be used for both intervention and enrichment. (After the overview at EdReady, go to Nebraska's site)
NROC: Complete courses in Secondary English and Mathematics.
Khan Academy: Videos, lessons and activities in a variety of subjects and for entire courses or test prep