A few tools of the trade which you may find useful for those occasions when you need to get your work done without too many distractions. Online tools and some offline tools included as usual.
Backboard - Feedback for uploaded media and collaborative text. Very shiny.
Plotbot - Nice screenplay tool; also a sandbox of shareable plots to play with.
Zoho Suite - Large array of collaboration tools; interfaces with Facebook.
full screen text editors
Web 2.0 aficionadoes will go to DarkCopy and try not to think about other tabs.
Offline - Windows users can use Q10 which is USB-friendly and free. I use this one myself for when I don't want to be distracted by Internet pretty-shinies and need to get flow. Mac users get WriteRoom while JDarkRoom works for anyone with Java.
Bartleby - Quotations and more; including Strunk's The Elements of Style.
iTools - Dictionaries, research desk. Collates good websites into one stop.
Online Books Project - Penn University site with lots of materials.
Project Gutenburg - The largest collection of online books I know of.
Rhymezone - Rhyming dictionary/thesaurus for those who need to.
If you know of any others which you've found good, post in the comments.