Here's some fantastic resources that can help teachers and students alike in understanding and exploring mathematics:

• Portal:Mathematics - Wikipedia's Mathematics Portal
 
• Khan Academy's Videos - An absolutely gigantic library of videos. Not just for mathematics! (Use jump to subject for math stuff)
 
• F.R.E.E.: Federal Resources for Educational Excellence - A resource repository of University and Federal projects for assisting education
 
• Math.com - Many tools, reference sheets, study tips and more. Targeted at students and Teacher (Look in the resources section of the sidebar)
 
• Teaching Today - A collection of articles concerningtechnology and pedagogy for teachers
 
• A Catalog of Mathematics Resources on the Internet
 
• Maths is Fun - Lots of explanations using simpler language and examples
 
• SOSMath - Easy to understand mathematics explantions, from algebra through college level courses
 
• Math for Morons Like Us - A site written by (one time) lay-people for lay-people (AKA The Uninitiated)
 
• ESL Mathematics - Resources for ESL and Mathematics