You may have seen photos with the 'little' or 'tiny' planet effect. There are quite a few iOS apps that will render this effect for you, but most have a cost associated with them while RollWorld is free and works quite well.
Import a photo from your camera roll, or set up your camera to take the photo "live". You can use any of six adjustments to roll the horizon into a tight ball, making it look like a little planet or asteroid.
RollWorld works just fine, and the rendering quality looks very good. I found photos that have something sticking up into the sky like a tree or a building give the best effects. The image I warped that is seen at the top of this page was taken at White Sands National Monument in New Mexico.
With the sliders, you can also do a reverse warp, rolling the horizon into a large ball with a hole in the center that is your sky. Your finished image can be rotated and zoomed to taste. Touch your finished picture and RollWorld reverts to the original photo.
The app also lets you shake your phone to get random settings, but I'd rather be in control of the final image. When you are done, you can save the image to your device, email it, or send it to Twitter, Facebook, wechat or Instagram.