Until now, the Google Cardboard virtual-reality headset has been largely an Android-only affair. Granted, there's a smattering of Cardboard-compatible iOS apps, but most of them were either offshoots of Android versions or rudimentary proofs of concept.
Roundme is an iOS app designed to take you on virtual sightseeing tours, and the latest update includes Google Cardboard support.
To get started, you'll need the headset, natch. You can buy a preassembled one from a supplier like Unofficial Cardboard. (Be sure to choose the Large model if you have an iPhone 6 or 6 Plus.)using inexpensive components, or go the shortcut route and
From there all you need is the Roundme app. Run it, then scroll through the available scenes (here known as Spaces). A great one to start with: Medieval Castle in Cesis, Latvia. (Downside: it will immediately make you long for higher-resolution lenses.) Tap it, turn your iPhone sideways, tap the Google Cardboard icon, then slip the phone inside your headset.
Shazam! You're whisked away to a fully panoramic landscape, with your view moving as you do. (Well, rotating as you rotate. Unfortunately, you're limited to whatever fixed position was used to create the panorama.)
To "visit" another place, you'll need to pop open the Cardboard and rotate the iPhone back to portrait orientation, which brings up Roundme's onscreen menus. The upper pull-down lets you browse three categories: Best New, Editor's Picks, and Recent. There's also a search option, and you can look at a world map to quickly jump to panoramas in a particular location.
Many, if not most, of them are positively dazzling. And some feature "portals" (which appear as onscreen bubbles) that can transport you to different locales within a given Space -- a neat way to explore without having to exit, choose another Space, etc. To move to a portal, you just keep it centered within your view for a few seconds until an onscreen meter fills up.
Last, but not least, if you sign up for a Roundme account, you can upload any panoramas you've shot with your iPhone. (You can also upload from your PC if you prefer to use more sophisticated tools.)
Pretty cool stuff, and all of it free. For any iPhone owner interested in fiddling with virtual reality, Google Cardboard or both, Roundme is a great place to start.