http://nlvm.usu.edu/en/nav/vlibrary.html - National Library of Virtual Manipulatives
http://www.vrml.k12.la.us/cc/math/vpfact.htm - Math skill links for PK-Algebra
http://www.321know.com/ - Sorted by grade (K-8)
http://www.oswego.org/ocsd-web/games/Mathmagician/cathymath.html - Math Magician contains practice with the four basic functions of math.
http://www.amathsdictionaryforkids.com/ - Math Picture Dictionary
http://www.everett.k12.wa.us/math/Home - Math games sorted by grade that practice the skills taught in Investigations/Connected math.
http://www.mathplayground.com – Math videos and games
http://moodle.fps.k12.me.us/course/view.php?id=19 – Connected Math
http://www.khanacademy.org – Khan Academy provides an extensive library of videos on a vast topics including math and science.