National Council of Teachers of Mathematics
Nctm: Figure This: Math Challenges for Families
NCTM offers sections on Families & School, Families & Math, Families & Homework, and more. Has links to brain-teaser problems ranging in skill from very easy to challenging. Click Take a Challenge or Challenge Index for...
Fundación Cientec
Cientec: Juegos Cooperativos
A small collection of 20 or so math brain teasers and word problems, in Spanish. Challenging for younger students -- they use math skills up to multiplication.