English Language related:

Dictionary.com
http://dictionary.reference.com
for their toolbar (...this is very convenient):
http://dictionary.reference.com/tools/toolbar/ 

Thesaurus.com
http://thesaurus.reference.com 

Reference.com (Almanac, Atlas, etc.)
http://www.reference.com 

Bartleby.com - the preeminent Internet publisher of literature, reference and verse
http://www.bartleby.com 

The Blue Book of Grammar and Punctuation
http://www.grammarbook.com 

Language Skills Handbook (see Grammar & Usage)
http://www.etsu.edu/scitech/langskil/ 

Garbl's Online Grammar Guides
http://home.comcast.net/~garbl/writing/grammar.htm 

Ogden's Basic English
http://ogden.basic-english.org/basiceng.html 

Learn English - Grammar Archive
http://www.learnenglish.org.uk/grammar/archive/mainindex_wide.html 

Grammar-Monster
http://www.grammar-monster.com/index.html 

Rice University: Ian Barland - (Some Ammo Against?) Grammar Police
http://www.cs.rice.edu/~ian/Manifestoes/grammarNazi.shtml 

Webgrammar
http://www.webgrammar.com 

Language Log: An internet pilgrim's guide to stranded prepositions
http://itre.cis.upenn.edu/~myl/languagelog/archives/000743.html 

Acronym Finder
http://www.acronymfinder.com 

...more to come later...