Dictionaries and Thesauri

English Language Dictionaries

• American Heritage Book of the English Language 
• Merriam-Webster Online
• Oxford English Dictionary

Foreign Language Dictionaries and Translators

Oxford Language Dictionaries
French, German, Italian, and Spanish to English and vice versa.

Thesauri

• Roget's II: The New Thesaurus

Specialized Dictionaries

• Acronyms and Abbreviations
• Online Encyclopedia of Graphic Symbols
• whatis?com (IT-related words)