Lingofy. Much more than a simple spell-checker. Also proofs text for usage, Plain Language and style issues. Free download.
See also Lingofy with The Associated Press style, sold directly by AP here:
ENGLISH DICTIONARIES AND THESAURI*
Oxford Dictionaries online in UK and US English; also a number of other languages including French, German, Spanish, Italian, Portuguese, Arabic and Chinese.
Macmillian Dictionary online has a dictionary, thesaurus and blog at this site.
Collins Dictionary also has a learner’s dictionary and other languages including French, German, Spanish and Italian.
*also spelled thesauruses
BLOGS ABOUT LANGUAGE
Oxford Dictionary – Language Matters
Macmillian Dictionary’s blog
World Wide Words covers all topics of English including history and etymology.
Wordsmith has a word of the day that comes by email:
The Associated Press (AP) has an extensive online site that covers all aspects of their style, primarily for newspapers.
The Chicago Manual of Style is used extensively by academic writers. The online version is located here.