I'm a bit late to this, but I've just discovered Daily UI. The whole idea behind it is to become a better designer in 100 days with daily challenges to improve your skills. You sign up and they send you a task each day to complete, except for weekends. You can also post your work up on dribbble or twitter, tag it as #dailyui and others can give you feedback on your work. It's a nice idea.
I've just gotten the first one, the task was to design a sign up page, they kind of left it open, whether you want to design a form, app screen, etc. I believe this sets the format of briefs, short, open and basically just a prompt for you to get started. With that, the tendency is to keep everything simple, which is quite freeing as a designer, as often you're limited by what the client says must be included, legal stuff, terms and conditions, etc.
Here's my attempt at Task 01: