http://www.ngf.org.uk/map/map.html
While you will likely find some useful educational sites through this Flash-based app, I’m pointing to it more because of the very nifty interface. It may seem disconcerting at first, but stick with it for a few more seconds. Mousing over the filters on the right-hand side will let you actually see how many sites will remain as a result (click the filter to then apply it), and clicking multiple filters will apply them one on top of the other. Thanks Trevor for pointing this out. – SWL