|Writing in the Sky by Shinichi Maruyama|
From the I Wrote a Novel, Now What? page at the NaNoWriMo site which might have more challenges added throughout the year (plus a few free contests). (For readers in the future, if you've stumbled across this page during November, the link probably won't work. A fresh page will go up early December.)
NaNoFiMo.org - National Novel Finishing Month (December). Goal: 30,000 words.
De-PlotWriMo - Plot Writing Month (December). Goal: Refine the plot arc of your first draft.
Varies or throughout the year
NaBloPoMo - National Blog Posting Month (Year-Round). Goal: Post every day for a month.
WriYe - (Year-Round). Goal: Set a word-count goal for the year and work towards it between January 1 and December 31.
48 Hour Film Project - (Varies; operates via tours around the USA, lasts 48 hours). Goal: Create a short film in 48 hours.
SciFiWriMo - Science Fiction Writing Month (Year-Round). Goal: choose a target word count and reach it in a month, writing sci-fi or fantasy.
750 Words (Year-Round). Goal: write 750 words a day. Includes month-long challenges.
JanNoWriMo - Goal: Write either 50k or your own word-count goal in January.
FAWM - February Album Writing Month (February). Goal: Write 14 original songs in a month.
NaNoEdMo - National Novel Editing Month (March). Goal: Commit to 50 hours of novel editing.
Script Frenzy - NaNoWriMo's sister challenge (April). Goal: Write a 100-page screenplay, stage play, comic book, or set of TV scripts in April.
RePoWriMo - Refrigerator Poetry Writing Month (April). Goal: Write poetry using only refrigerator poetry magnets.
April Fool's - (April). Goal: Set a word-count goal for yourself and fulfill it by the end of the month.
National Picture Book Writing Week - (First week of May). Goal: Write 7 picture books in 7 days.
NEPMo - National Epic Poetry Month (May). Goal: Write 5,000 lines epic poem in May.
SoFoBoMo - Solo Photo Book Month (Between May first and June 31). Goal: Create a solo photo book within 31 days.
Story A Day - (Throughout May). The only rules: Write a story a day. Finish them. Note from me: this could easily be harder than NaNo if your stories are more than 1667 words each! :-) If you'd like to keep word count under control you might want to check out the various forms of Flash fiction.
ComiKaze - Create a 24 page comic in 24 hours. Participants pages are posted at the Pulp Faction forums.
WriDaNoJu - Write a Damn Novel in June (June). Goal: Write 50K in the 30 days of June. It's perfectly situated six months from November so you have optimum time to prepare for WriDaNoJu and NaNoWriMo.
SoCNoC - Southern Cross Novel Challenge (June). Goal: Write 50,000 words of fiction.
JulNoWriMo - July Novel Writing Month (July). Goal: 50,000 words for a new or unfinished manuscript.
AugNoWriMo - August Novel Writing Month (August). Goal: Write a novel in one month.
3-Day Novel Contest - (September). Goal: Write a novel in three days. They've been doing this since 1977. So cool!
SeptNoWriMo - September Novel Writing Month (September). Goal: Set a word-count goal and edit, write, or edit and write throughout the month of September!
24 Hour Comics Day - (Date changes annually, lasts 24 hours). Goal: Draw a 24-page comic in one 24-hour period.
GothNoWriMo - Gothic Novel Writing Month (October). Goal: Write a gothic novel in October.
NaPlWriMo - National Playwriting Month (November). Goal: Write a play in one month.
NaNoMango - The manga alternative to NaNoWriMo. Either 30 penciled, 15 inked, or 10 completely finished pages of a comic book in 30 days.
PiBoIdMo - Picture Book Idea Month (November). Goal: Write 30 ideas in 30 days.