I have been wanting to participate in the worldwide sketchbook project created by the Brooklyn Art Library for some time, and finally in November I signed up to create a sketchbook. How it works is that you purchase a sketchbook from The Sketchbook Project with the option of having it digitized, and you fill it up when you receive it using one of the themes or with whatever you like, following the rules and size limitations.
After completing your sketchbook, you link it up to the library online and mail it back to be included in their permanent collection. If you have purchased the digitized option, your sketchbook will be scanned and made available online for others to view. The sketchbooks can be checked out and viewed in person at the Brooklyn Art Library or on one of their mobile sketchbook tours around the country.
Here are a few pages from my sketchbook entry with the theme "All about me". The fully digitized version can be viewed here.