Artrage isn't a web tools as such but it's so good I just have to keep it on the site. It's a lovely drawing program where you can select paintbrushes, colours and have a go at bringing out the artist in yourself or your students.

Something like this would be brilliant on an iPad. Whilst I know there are other programmes that will do much more this has a certain delightful character and is so easy to use. There is a free version but I don't know how much longer for. Enjoy it while you can!