This is an art challenge running every year from June 1st to August 31st. You can take part in one, two, or three months.
What? Draw something every day to a topic you chose yourself.
Where, how to take part: Sign up at Livejournal or Dreamwidth. There is a Tumblr, too.
This is a gamified art challenge running from July 1st to August 1st
What? Participants are split into two teams. You score points by drawing characters belonging to a member of the other team. The team with the highest score wins
Where, how to take part: Sign up on the official website, artfight.net
This is an art challenge running annually from August 1st to 31st
What's the challenge? Draw fat people, ideally one per day
Where/How to take part: Post anywhere, using the tag “AugustAdiPOSE”
This is a writing challenge running each month.
What's the challenge? Write at least 500 words each day.
Where, how to take part: Post on Twitter with the monthly changing tag, and insert your progress in a communal spreadsheet. More info at writingchallenge.org
This is a weekly flash fiction writing challenge.
What'S the challenge? Every Thursday an idiom and a target wordcount is posted for people to write a piece of flash fiction.
Where, how to take part: Prompts are posted at thimblefulthursday.wordpress.com, and posting links to your prompt fills in the comments there is appreciated.
This is a roughly monthly art jam running from one Friday to the following Sunday, noon to noon Alaskan time. There is a focus on fantasy themes.
What's the deal? You can leave prompts for other people to use, and/or draw sketches based on other people's prompts. You may not spend more than 1 hour per sketch. (Finishing it later is cool, though.)
Where, how to take part: Sign up on the official website, www.ellenmilliongraphics.com/sketchfest/