The first is just called "NASA". It's a great place to start. The home screen is a shot of our solar system. Click on any object and you get detailed information about that planet, star, or moon. Along the top of the screen are some great options: launch services, maps of NASA centers, NASA's launch schedule, and details about satellites. The bottom has several options for news, NASA TV, videos, tweets, and my favorite, NASA's radio station (Third Rock).
Another app to try is NASA Viz (Vizualization Explorer). This apps offers the stories behind NASA's exploration. It highlights new discoveries through the satellites and far-reaching exploring vehicles. Each slide offers details about the mission, images, and videos. I especially enjoyed the story called "Cruising with Curiosity", the story behind the Mars explorer.
Finally, check out NASA TV. It is an app that offers real-time viewing of NASA TV (which can be pretty boring sometimes because you watch a lot of control centers). The best part is the on-demand viewing. Choose to watch short clips of testing videos, interviews, and updates on projects in development. A nice way to keep up-to-date with NASA.
I hope you get a chance to try these out. Did I mention they are all free? Happy exploring!