In his book, Drive, Daniel Pink describes what he calls "Fed Ex Days." I'd call them Google Days, but the premise is the same. Employees are given 20% of their work time to work on whatever projects interest and excite them. In the Google world, this 20% time has resulted in lots of new products for Google, like Talk, Gmail, to name a few.
Pink advocates for such days in a school setting. I've heard of various people putting it into action at the school wide level, but just discovered a teacher who is documenting how it's working in his classroom. It just so happens that this particular teacher is implementing this in an 8th grade classroom, with the rules of it must relate to the topics considered "Social Studies" and they must be using skills included in the curriculum. Check out 20 Percent blog.
I'm very curious what they come up with, how it works out, how it impact the teacher's ability to "get through" his curriculum, and I'm particularly curious how I might implement it on a block schedule (because while I have no problem giving up a 40 minute period a week, giving up 83 minutes every two weeks seems daunting!)