Flightradar24 for Mobile Devices

Realtime tracking of aircraft from your iOS or Android devices. Identify planes flying overhead in the augmented reality view by simply pointing your phone to the sky.

https://www.flightradar24.com/apps

Stellarium for PC and Mobile Devices

Stellarium is a free open source planetarium for your Computer and Mobile Devices ($2.99 on Mobile). It shows a realistic sky in 3D, just like what you see with the naked eye, binoculars or a telescope. Augmented Reality is also available for Mobile Devices, just point your device at what's up there...

http://www.stellarium.org/

The UFO Detector is a free software that allows anyone to monitor the sky for UFOs day and night. If a UFO is detected the software automatically records a video of the event in the best possible quality.

http://ufoid.net/