Starting on Monday, Lynda.com is hosting a 21 day drawing challenge taught by Von Glitschka. The idea is to draw everyday and that pencil & paper drawing is important, even (or especially) in digital art. It also looks like this challenge is free if you like lynda.com on Facebook. I'll be checking it out and I'll let you all know how it goes!
Check out the details here: http://www.lynda.com/articles/21-day-drawing-challenge.
Update: June 2015
I don't know why I never got around to posting images here. I posted them on my personal Facebook and never thought about them again. Here's an image containing most of the scribbles I made for the 21 Day Drawing Challenge. I honestly don't remember what the themes were at this point....