Writing and Editing Resources
Dictionaries
Merriam-Webster
Merriam-Webste is probably the best-known dictionary of American English. It can be searched for free at www.merriam-webster.com. Search results include not just word meanings, but also examples of usage and history of the word in question.
Chambers
Chambers is a dictionary of British English. It is said to include more unusual words than other dictionaries of similar size, and is valued by many for the etymological information in each entry. The dictionary can be searched for free at www.chambers.co.uk.
Thesauruses
Although probably most used in the writing of fiction and general non-fiction, thesauruses can also be very useful in scientific and technical writing.
Fortunately both Merriam-Webster and Chambers offer free access to their thesauruses. For the curious: a quick glance at Chambers dictionary will reveal that 'thesaurus' comes from the Greek thesauros (treasury) and that, yes, 'thesauri' is also an acceptable plural.
The Moby Thesaurus is the largest English-language thesaurus. It can be searched for free here. It defines 'synonym' rather broadly; as a result, while Merriam-Webster and Chambers list respectively two and eight synonyms for 'dictionary', the Moby Thesaurus lists 38. It is perhaps best reserved for occasions when searches of the other thesauruses are unsuccessful.