My old design college has made it’s own app in conjunction with developers Semi Strange. It’s brilliant.
It’s full of technical info and reference for designers, everything from typography structure and design principles to print finishes and comprehensive creative suite shortcuts.
I really can’t recommend this enough, it’s an invaluable resource for any designer (with an iPhone)!
http://shillingtondesignblog.com/posts/271/v/grid/ViewPosts_page/1