If you were a kid during the Reagan era, you have fond memories of staring into a View-Master. The plastic toy looked like a modified set of binoculars, but allowed you to see all kinds of interesting stereoscopic 3D photos on rotating cardboard discs.
If you own an iPhone, you can get a View-Master-like experience, complete with the eyepieces, using one of two accessories. Hasbro's $8 MY3D viewer lets you see 3D content on the screen when you stick your iPhone inside it and run a custom app. The $59 Poppy 3D takes 3D a step further, allowing you to both capture and view stereoscopic 3D on your iPhone. Some older Android phones such as 2011's LG Thrill 4G provide a glasses-free 3D experience, but this technology has fallen out of favor in recent years.