Jenny Eather has a great animated, interactive primary school dictionary.
WikiEducator.org is in the early stages of developing a resource for Mathematics projects .
Information from most of the Wiki sites (hopefully WikiEducator.org will be the exception) tends to have too much technical detail for use in primary and secondary school mathematics programs. Developing conceptualization, which is a much harder task than providing definitions, is what Wiki sites need to do more of to help teachers and self motivated learners to learn mathematics. That said the following are offered as starting points in the Wiki world:
Wikipedia:
1. List of basic mathematics topics
2. Mathematics
3. Mathematics history and content
Wikiversity:
1. Primary school mathematics
2. High school mathematics
Wiktionary:
1. Arithmetic
2. Algebra
3. Mathematics