NaNoFiMo.org - National Novel Finishing Month (December). Goal: 30,000 words.
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 or stage play in April.
RePoWriMo - Refrigerator Poetry Writing Month (April). Goal: Write poetry using only refrigerator poetry magnets.
NEPMo - National Epic Poetry Month (May). Goal: Write 5,000 lines epic poem in May.
SoCNoC - Southern Cross Novel Challenge (June). Goal: Write 50,000 words of fiction.
WriDaNoJu - Write a D*** 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.
SoFoBoMo - Solo Photo Book Month (Between May first and June 31). Goal: Create a solo photo book within 31 days.
JulNoWriMo - July Novel Writing Month (July). Goal: 50,000 words for a new or unfinished manuscript.
24 Hour Comics Day - (Changes annually, lasts 24 hours). Goal: Draw a 24-page comic in one 24-hour period.
48 Hour Film Project - (Varies; operates via tours around the USA, lasts 48 hours). Goal: Create a short film in 48 hours.
Book in a Week - (Begins on the Monday of the first full week of each month, lasts one week). Goal: Write a novel.
April Fool's - (April). Goal: Set a word-count goal for yourself and fulfill it by the end of the month.
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!
NaPlWriMo - National Playwriting Month (November). Goal: Write a play in one month.
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.