General
Channel4HomeworkHigh
(site where your questions are answered by professionals for free)
BBCSchools
(information & support that's closely tied in with the national curriculum used in UK schools)

Reference
Wikipedia
(Encyclopaedia written by the people for the people.  One of the most positive things to come out of the world wide web)
Encyclopaedia
(look up anything you need to find out about)
HowStuffWorks
(well written explanations of how things work -zips, engines, fireworks, brains… everything!)
BrainPop
(collection of video clips covering a wide range of educational topics with quizzes to do while waiting for the movie to download)
KidInfo
(well organised US site covering many school subjects)
MultnomahLibrary
(an alternative to KidInfo)

English dictionaries/databases
RhymeZone
(database with definitions, rhymes, synonyms etc. for any English word)
Dictionary
(type in a word at the top of the screen and click 'Look it up')
Thesaurus
(to find other words with similar meanings, type a word at the top of the screen and click 'Search')

Other languages
GoogleTranslate
(type an English word or phrase in the box, select the language you want then click 'Translate')

Photo library
Flickr
(thousands of photos to view)