“Any creation of a mockup through this website happens at your own risk. Note that your mockups are not protected in any way other than by a non-obvious URL. … This is an alpha version. It may disappear at any moment. Your mockup may be deleted at any moment.”
Or you could use Balsamiq, which has been doing the ‘sketch’ look mockups for a while, but runs on your machine so you’re not likely to lose them!
- D N S - 
“Any creation of a mockup through this website happens at your own risk. Note that your mockups are not protected in any way other than by a non-obvious URL. … This is an alpha version. It may disappear at any moment. Your mockup may be deleted at any moment.”
Or you could use Balsamiq, which has been doing the ‘sketch’ look mockups for a while, but runs on your machine so you’re not likely to lose them!
http://www.balsamiq.com/products/mockups
How about an app for designing apps!