I use balsamiq mockups:
http://www.balsamiq.com/products/mockups
Really nice and free...
You really can make nice looking to be apps with this and it is cartoonish but very slick.
Check out how the google, youtube, and the big boy sample apps here: http://www.balsamiq.com/products/mockups/examples#boogle
If you want to share your designs you can look at mocking bird: http://gomockingbird.com/
But to be quite honest I think you will love using balsamiq for putting together something on the fly to show to other co-workers / clients.
If you want something free and built into say firefox try pencil
