Truly Innovative iPod Touch Apps

I have been fortunate enough to have secured funding for four iPod Touch devices for my classroom, (read the back story here), and I am looking forward to putting them to work with students very soon. I already have a few great apps in mind, but for those of you that have used iPod for a while, which apps do you consider to be the truly great ones? Which are innovative enough to really alter the way you might normally teach? Which ones are indispensable, universally popular or just tailor made for the classroom? All ideas welcome.


Thanks.