The National Council of Teachers of Mathematics Illuminations website is full of activities and lesson plans aligned to NCTM standards. I’ve previously reviewed some of those activities and lessons here. This evening I’d like to call your attention to NCTM’s collection of 724 links to mathematics lessons and activities that are not hosted by NCTM. NCTM’s weblinks collection is reviewed by an editorial board. The collection is divided into five categories; Numbers & Operations, Algebra, Geometry, Measurement, and Data Analysis & Probability.

Here are some related items that may be of interest to you:
A Maths Dictionary for Kids
Five Sources of Fun Mathematics Games
Ten Problem Solving Games for K-8 Students