Ask Oxford - search the Oxford English Dictionary and other handy tools
Chambers Reference Online - dictionary and thesaurus
Visual Thesaurus - try typing in 'cake'!
OneLook Reverse Dictionary - for a list of related words and phrases
Bible Gateway - search and compare different Bible translations
Word Spy - investigate new additions to the English language

Some links you may find fun:

Numberwang - 'I'm sorry, that is not a vegetable'
Genius - clever inventions and cunning schemes