Note: When I started my first website for teachers in 2004, one of my intentions was to share links to other useful websites. At the time, there were a few great sites and new ones cropping up all the time, but nowhere near the variety available now. Over time sites change or disappear, and links go down. So I'm moving the lists of my favorite websites to blog posts, to show that as of the date of this post, these links all work and connect to some resources I've found to be helpful.
Math Resources: 
A+ Math (Grades 28) 

Ask Dr. Math (Grades 212) 

Kids’ Place Mathematics from Houghton Mifflin (Grades K3) 

Math Forum: Problems of the Week (Grades 412) 

Math World (Grades 812) 

Muliplication.com Classroom Games (Grades 38) 

That Quiz (Grades 110) 

Websites for Teachers: Math
I really dig MathWorld and I use it alot. You are missing a few key math sites that I’m surprised about. Like you said it might be that these pages have changed over time. Right now, PurpleMath and MathWorksheetsLand.com and true staples in my class and in my building.