Dictionaries
Dictionary.com: English & foreign language dictionaries including a translator.
Cambridge Dictionary: Good sample sentences.
Macquarie Dictionary: The concise version.
Internet Picture Dictionary: Suitable for lower levels.
Oxford Advanced Learner's Dictionary: Good overview of patterns and collocations
Longman Dictionary of Contemporary English: Includes pictures
MapMyWord Dictionary: Looks at word families.
Online Thesaurus: Lists possible synonyms.