Some of these are nano-specific, but many of these are useful to all kinds of writing. If you have some resources to add, leave them as a comment to this post.
http://nanowrimo.org/ - the NaNoWriMo site. This is an obvious one.
http://writeordie.com/ - Write or Die - for when you need some tough love. Set a word count goal and a time limit. If you don't hit it, bad things will happen.
http://www.nowhitespace.org/writing/ - Writing Cheat Sheet - a downloadable pdf with many character, plot, and story resources all in one handy place
http://www.writersdigest.com/online-editor/how-to-prepare-for-national-novel-writing-month-nanowrimo - Writer's Digest - how to prepare for Nano
http://www.tengrrl.com/tens/011.shtml Character journal topics
http://www.seventhsanctum.com/ Seventh Sanctum - many, many generators
http://surnames.behindthename.com/ Behind the name surnames - Surnames and their origins and meanings
http://www.behindthename.com/ - Behind the name - first names - First names and their origins and meanings
http://www.openoffice.org/ - Open Office - provides Microsoft Word like functionality but it's free
http://kristina-bjoran.suite101.com/5-fiction-writing-prompts-for-plot-a152987 - Plot development exercises
http://www.readwritethink.org/files/resources/interactives/plot-diagram/ - Plot diagrammer
http://iconoclasticwriter.com/fiction-writing-plot-development-storyboards/ - Fiction writing storyboards - downloadable storyboards for outlining a novel
http://funstuff.pantomimepony.co.uk/writers-plot-ideas.htm - Plot generator - there are also some other useful generators on this site
http://grammar.ccc.commnet.edu/grammar/ - Grammar - Want to brush up on your grammar or have a specific question? Great grammar resource.
http://allfreelancewriting.com/word-count-tracker/ - Simple word count tracker
http://www.languageisavirus.com/ - Language is a Virus - prompts, generators, and many varied resources
http://www.spontaneousderivation.com/2008/02/06/6-word-meters-and-trackers-for-the-word-count-obsessed/ - Six word count meters
http://cte.jhu.edu/techacademy/web/2000/kochan/charactertraits.html - List of character traits
http://anthonyowens.wordpress.com/2007/10/17/100-character-quirks-you-can-steal-from-me/ - 50 character quirks
http://community.wizards.com/go/thread/view/75882/19873978/1001_Character_Quirks - 1001 Character Quirks
http://www.maxinethompson.com/Apr06.html - Character development exercise
http://tinyurl.com/nqnnos - 100 Character Development questions - excellent in-depth character interview