Google Cardboard: Simple Virtual Reality headsets to be attached to a mobile phone for experiencing VR media. Simply attach the cardboard to your phone and play the media while using the viewer. Paid. (Build your own Cardboard viewer for free by following these instructions.)
Paid
audioBoom
audioBoom: create, edit, and share audio files online via a mobile app or your browser. Deeming itself as “radio reimagined,” audioBoom functions very much like YouTube, but with a focus on audio. Premium features include the ability to record and share audio tracks that are longer than 10 minutes, as well as create a “broadcasting” channel. Premium.
PDF Expert
PDF Expert: a robust annotation tool for Mac desktops, iPads and iPhones. Offering a multitude of annotation options (highlighter, stamps, text, note, underline, shape, etc) on a wide range of files (PDF, document, spreadsheet, image, video, audio, etc). Offers seamless synchronization with cloud storage platforms such as DropBox and Box, and a Markup feature for PDF files with powerful editing abilities. Paid.
iAnnotate
iAnnotate: an annotation app for your iPad or Android device. Available for purchase on the App Store.