One of my RWA online chapters (actually two of them) are holding writing challenges right now. Fall seems like a popular time with NaNoWriMo probably the most well know being held every November. One of my chapters is holding a two month challenge with lots of classes, pitches to editors and agents, and writing sprints for it's members throughout September and October. My other chapter is doing something similar to NaNoWriMo except you can work on whatever you want and you can be writing, editing, marketing, etc. It's devoted to all writerly activities
I haven't been able to really participate that much in my first chapters challenge though I did do it last year. I'm helping to organize my second chapters challenge this year. I've always liked these challenges. They can teach you better writing habits and force you to shove your inner critic aside.
How about you? Do you participate in writing challenges?