The National Library of Virtual Manipulatives, developed at Utah State University, is very popular among math teachers at my school. This free site supports math students from kinder through 12th grade, with plenty of online exercises in Number and Operations, Geometry, Measurement, and MUCH more! Each major concept is organized by grade (K-2, 3-5, 6-8, 8-12); once you click the appropriate tile, a long list of applicable practice items from which to choose will be listed.
CC Connection: This site covers the vast majority of Math core standards. Lots of ways to provide your students opportunities to see and practice various important 'core' math skills, using online manipulatives.