Wikipedia makes a natural reference resource: It's free, easily accessible, and has a much greater breadth of coverage than most reference sources. The downsides of using Wikipedia in a classroom setting include, of course, the potential for inaccurate, incomplete, or vandalized material and, for young children, possibly inappropriate material.

One possible solution to these problems is to use Wikipedia article selections on CD or DVD, as summarized in Appendix A, Reusing Wikimedia Content. Two of these selections were designed specifically for schools, one particularly geared toward the UK National Curriculum. You can browse the articles on the DVD, which were edited and checked for suitability, at http://schools-wikipedia.org/.

For children or English-language learners, the Simple English Wikipedia may also be appropriate. Available at http://simple.wikipedia.org/, this site aims to cover the same topics as the English-language Wikipedia, but in a simpler version of English. The breadth of the site is not nearly as large as that of the English-language Wikipedia, however, and many articles are still stubs.