Search Engines

Especially for primary students
http://www.dogpile.com
http://www.askforkids.com
http://search.yahooligans.com

http://www.kartoo.com/ (guided search)
http://msxml.webcrawler.com
http://search66.com (very clear website description)
http://www.mooter.com (uses mind mapping techniques to refine your search)

Online Encyclopaedias

Encyclopaedia Britannicia online school edition
http://school.eb.com/storepassword

http://en.wikipedia.org
http://www.factmonster.com/encyclopedia.html

Online Dictionaries

http://www.websters-dictionary-online.org/ (multi language)
http://www.answers.com
http://encarta.msn.com/encnet/features/dictionary/dictionaryhome.aspx

Online Thesaurus

http://encarta.msn.com/thesaurus__/thesaurus.html