Readers of the forum might like to explore the Open Directory Project mathematics pages at http://dmoz.org/ The ODP is the largest, most comprehensive human-edited directory of the Web, constructed and maintained by a vast, global community of volunteer editors.

It is hierarchically organised: school-level mathematics resources are listed at http://dmoz.org/Kids_and_Teens/School_Time/Math/ and college- and professional-level resources at http://dmoz.org/Science/Math/

The ODP relies on submissions from the public, which are reviewed by volunteer editors (and yes, I am one of them).