This could be the most useful app you ever put on your BlackBerry. It works great on my Javelin 8900 and its a little addicting. I enjoyed Etch-A-Sketch ing back in the day and this simple little app brings back all those wonderful memories.
You can hit Space to clear what you've sketched and it vibrates for effect. You can also take a snapshot of your masterpiece by pressing Enter. Although I'm not sure where the snapshots are saved yet.
Here's a link to the download page: http://store.blackberryrocks.com/product.asp?id=30406&n=SketchIt
It says it will only be free for a little longer, so get it now and get sketching!