Alright, I figured since this is Comics & Drawings this would apply better than just the Arts & Crafts forum, but let me know if I'm wrong. A few forums I frequent have this, and one in particular still amazes me everytime I see it.
But here's the basic idea:
1. You make a drawing/painting in your favorite program (photoshop/paintshop pro/painter/oc/whatever).
2. You keep your work to a time limit, basically an hour or less.
3. You keep doing this - daily would be nice, but some of us don't have that much free time, and you post each piece and people can admire it or critique/comment.
Over time you should be able to see an improvement in your skills - not to mention get ideas and learn things from all the other people posting.
While I love illustrator and what not, vector based images and the like that aren't sketched to begin with shouldn't be included. Try not to limit yourself to the same thing every time and be sure to try out different things. If you always draw comic form, try realism/proportional drawing. Try abstract, minimalist, etc etc.
Here's something I did earlier briefly when I was bored. Took me around 67 minutes or so. Since the point is to keep it timed, this is as far as I'm going to go with this picture.
Hopefully this doesn't die and we can see a few more members' work. Hell, it'd be nice if Jeph even tossed something in.