I do get this question, and think these links give some great information:
http://www.google.com/search?hl=en&hs=znS&lr=&client=firefox-a&rls=org.mozilla:en-US:official&oi=defmore&defl=en&q=define:Unschooling
http://www.naturalchild.com/guest/earl_stevens.html
http://www.holtgws.com/index.html
http://www.johntaylorgatto.com
http://www.unhinderedliving.com/unschooling.html
http://www.unschooling.org/
And while not exactly "unschoolers", we base a lot of our methods on The Moore's:
http://www.moorefoundation.com/about.html