Clarkness.com: Beginner Reader and Math eBooks. All books are free. Books are also available in Spanish. 

BeginningReads: Downloadable books and beginner reading lessons.

National Library of Virtual Manipulatives: Digital library with math activities for K-12. Includes topics such as numbers, operations, algebra, measurement, and more!

Cool Math Games: Practice math concepts while playing games.

Free Math Apps: Website that includes lists of free math apps.