Kansas Association of Teachers of Mathematics (KATM) - Lots of resources for implementing the common core. Site was recommended at Math Solutions Workshop. Scroll down to the flipbooks, which are detailed and printable explanations of each standard by grade level (K-8) with good examples.
K-8 Math Wiki to UnPacking the Standards - K-8 math wiki created by teachers with the help of OCM BOCES. Easy to navigate, well-organized site with explanation of standards by grade level and some links to examples.
Center on Instruction - Gateway to cutting-edge collection of scientifically-based resources on instruction. Free resources in literacy, math, RtI and more.
K-5 Math Teaching Resources - A collection of (printable) free resources, math games, and hands-on math activities aligned with Common Core Standards and broken into broad categories of Number, Geometry, and Measurement/Data.