Connecting Mathematics AKA http://thesausrus.maths.org must be mentioned since I've put so much work into this site over the past two years. It's an open source, open content thesaurus targeted at school/early university level with some neat graphical views of how mathematical concepts relate to each other. Some good visualisations too - especially in the VRML section.
It's also international - 9 languages currently.

If you're interested, leave a message for me on the forum. We could do with a new batch of supporters now that our EU funding has run out.