Underground Mathematics
This site is beginning to get some seiously impressive feedback.
Written by a team who have thought extremely hard about task-creation,
the open investigatory activity is celebrated here.

Nrich
A brilliant collection of materials for all ages, with a strong A Level section that is carefully indexed.

Waldomaths
Ronald Barrow has assembled here some wonderful Java applets, along with some videos and other materials.

Stem Mathematics Library
Collected together here in a national library are the best resources freely available for STEM subjects.
The link here takes you to the 16+ area.

TES Teaching Resources
This site hosts a huge array of resources, some to pay for and others that are free.
There is a way for users to give feedback, so good resources are easy to find.

Technology for Secondary/College Mathematics
If you wish to find a risp-friendly site,
then it is bound to be found here. A collection of links maintained by Douglas Butler:
why write your own links page when the ultimate links page has been compiled already?