Until now, I’d used Google Toolbar + Dictionary.com to look up definitions of words rapidly (I type the word in Google Toolbar, click Search, then click the word itself in the results page which loads up dictionary.com).
Looking around on the Internet for more detailed meanings of words, I stumbled upon WordNet, a project that has a very useful approach to word searches. You can type a word to find a meaning as one would expect, but the clever part is with what you can do next: a dropdown menu presents a list of futher searches based on your word, to show Synonyms, Hypernyms and Hyponyms, and a number of other searches. Hypernyms and Hyponyms are particularly interesting alternatives to Thesauri.