Book by Debbie Diller:
http://www.stenhouse.com/shop/pc/viewprd.asp?idProduct=9282
Blog with math center tips (K-2):
http://primaryclassroomsareoceansoffun.blogspot.com/2011/04/guided-math.html
Marcy Cook ideas (K-2):
http://www.marcycookmath.com/index.php?option=com_virtuemart&Itemid=14
K-5 Sample Ideas for Math:
http://www.k-5mathteachingresources.com/kindergarten-math-activities.html
Center Ideas (4-5):
http://www.thefutureschannel.com/hands-on_math.php
Center Ideas (4-5):
http://www.uen.org/core/math/index.shtml
Excellent Math Ideas for all grades:
http://www.mathwire.com/
http://www.figurethis.org/
http://www.khanacademy.org/