Free! MATH CHIMP is a curated collection of videos, 200 or so games and printable worksheets that can supplement math instruction in grades 1 through 8. The materials are created by a variety of groups (many of the videos, for instance, are from the Khan Academy). The added value here is that all resources are aligned with Common Core State Standards (in math). Users who register with the site can rank the games, videos and worksheets with one to five stars to indicate what they like the most.