Space Place Prime for iPhone 3GS, iPhone 4, iPhone 4S, iPhone 5, iPod touch (3rd generation), iPod touch (4th generation), iPod touch (5th generation) and iPad on the iTunes App Store.
via Space Place Prime.
Space Place Prime is a free app distributed by NASA. It contains all kinds of cool articles, videos, and images put out by NASA, but completely intended for elementary level. However, when I say elementary level, most of it would probably meet the needs for second grade on up.
One drawback of this application is the organization. I wish there were some sort of search option so that the contents of it could be easily found. At this point, it seems pretty random, and sometimes it is difficult to navigate through. Other than that, the contents are good and would be good for students to use as a supplemental science resource.