Know and love:
Living math - math reading lists, how fun is that?!
Math Dude -math podcast
Math Candy - need I say more?
Math Enhancement Program - free math curriculum
Funny Math - some fun math facts and tricks
Kahn Acadamy - Free math videos (also science, humanities, and other..)
Vi Hart's Math Doodling - a recreational mathematician shares her thoughts
Project Euler - use computer programming and/or spread sheets to solve complex math problems
Art of Problem Solving Videos - math help for high school level math
Haven't used as much but still interesting:
Math Circle - chewy math questions and problems
National Centre for Excellence in the Teaching of Mathematics - some interesting links about teaching math
Math Playground - Arithmetic games, my kids don't get into this sort of things, but yours might.