"Flinto strikes the perfect balance between fast prototyping and realism.
Prototypes run on iPhone or iPad and can be shared with anyone." Looks very promising.
Easily preview your designs and icons on your iPhone and iPad.
A beautiful Wikipedia reader for your iPhone and iPad that visualizes the connections between articles.
Upload your icon and see quickly see it in various iOS devices and settings.
CreativeApplications.Net is your daily dose of inspiring apps for Mac, PC and iPhone... Some good stuff on here.
Supply a 512x512 PNG and this app will output all relevant sizes.
Name your layers correctly and this free Photoshop plugin will sort your button states and device resolutions for you.
Print out a phone or tablet PDF to help with sketching that app. Thumbnails are a bit wonky so no guarantees on the prints.
Like to mocking up your apps in keynote, here's a fantastic collection of templates to help you out. And free (pay with a tweet).
Meaningful Transitions deals with the use of animations in the user interface. Pretty good for a student project.
iPads, iPhones, browser windows and various popular grid systems. Print these out and colour them in.
A well categorised mobile app gallery for your viewing pleasure.