This is "a collaboratively edited question and answer site for linguists, etymologists, and serious English language enthusiasts. It's 100% free, no registration required (Welcome! 2011.02.02).
"The best part about Dropbox as a collaborative tool, is that two people can edit the same file simultaneously. In that situation, Dropbox saves two distinct copies of the same files, each with a person's edits to ensure that both edits are preserved" (3. Be Smarter About Collaborating On Projects, ¶3, 2011.02.23).
"Originally written by Janelle Ward for The Broker, and first published on August 5th, 2009 as Rewriting Research" (footnote [URL revised, PB, 2011.02.21]), this article covers the following outline:
Academic BloggingBlog Types and PurposesWho Are Academic BloggersBlogging RewardsCollaborative Writing and WikisPros and Cons of Using WikisWikis['] EffectivenessWeb 2.0, Web 3.0 and BeyondThe Growing Use of Online Tools
"Text 2 Mind Map is a web application that converts texts to mind maps.
It takes a structured list of words or sentences, interprets it,
and draws a mind map out of them" (description below working display, above Hints for using it).
"TinEye is a reverse image search engine. It finds out where an image came from, how it is being used, if modified versions of the image exist, or if there is a higher resolution version.... It is free for non-commercial searching" (home page, 2011.02.14), and there is a Firefox plug-in (Introducing TinEye [video]).
"a cloud based document comparison and redlining tool making it easier to share, delegate and collaborate" (Document comparison in the cloud, ¶1, 2010.02.14)
For learners of English, sports posts in various categories: adjectives, grammatical expression[s], grammatical word[s], idioms, phrasal verbs, and the difference[s] between words (Categories, 2011.02.02)
Thanks to Yuly for pointing out this powerful toolkit allowing wildcard searches and including a reverse dictionary. OneLook contains "19,044,271 words in 1062 dictionaries indexed" (home page, bottom line, 2011.01.28).