Google Cardboard is a free system for viewing 360° immersive photos using many smartphone apps and an inexpensive viewer, many of which are made of cardboard, thus the name. On this page you’ll find some free resources available in the iOS Apps Store (the main link in each entry) and Google Play Store to get started with Cardboard.
Cardboard Viewers – If you don’t already have a VR viewer for Cardboard, start with this page that shows a variety of viewers at many different price points.
Expeditions – This is a system, not just an app. Google has more than 200 short 360° tours from all over the world and you should read all about the equipment required and the process.
Street View – Another Google app that allows you to choose from thousands of Street View locations to explore. This app can also be used to create Photo Spheres (more information about that is here) that can be added to Google Maps. For Android
You can find many more VR apps (new ones are posted every week) by searching in your favorite app store.
360° Video on YouTube– You can also view immersive video in Cardboard. Select from a growing collection of immersive videos from both professional and amateur sources on YouTube.