Merriam-Webster Online - This online source includes Merriam-Webster's Real Time Words, which consist of unedited entries randomly selected from millions of searches on Merriam-Webster Online that make the Top Ten List of the most frequently looked-up
Cambridge Dictionaries Online - This online dictionary ranges from intermediate to proficient level; it is clear, up-to-date, and easy to use.
Online Etymology Dictionary - The basic sources of this work are Weekley's "An Etymological Dictionary of Modern English," Klein's "A Comprehensive Etymological Dictionary of the English Language," "Oxford English Dictionary" (second edition), "Barnhart Dictionary of Etymology," Holthausen's "Etymologisches Wörterbuch der Englischen Sprache," and Kipfer and Chapman's "Dictionary of American Slang."
Diccionario de la lengua española - This is the lexicographic reference work of the "Real Academia Española"
Merriam-Webster Online Thesaurus - pulldown "Dictionary" at the top to select the Thesaurus instead.
Roget's II : The New Thesaurus - 3rd ed. - contains 35,000 synonyms and over 250,000 cross-references.