The prevailing view in recent years is that our post-keyboard future is very likely to be informed by these kinds of wearable "always on" input devices: rings, bracelets, smartwatches, or everyone's favorite Next Big Thing, smartglasses.
Smartglasses may be the most promising wearable input device in that their proximity to the eye -- our primary sensory apparatus -- opens up a lot of options. Most smart glasses employ OHMD (optical head-mounted display) technology, which essentially bounces a kind of picture-in-picture display into your field of vision. Combined with other alternate input device solutions, like Google Glass' voice command system, OHMD can provide truly mobile hands-free viewing.
Meanwhile, companies like Tobii in Sweden are deploying eye-tracking systems that are already in use as computer input device solutions. By tracking your eye movements and determining where your gaze lingers, such systems can be used to execute commands with traditional workstation setups, through your smart glasses, in your car or even in old-school video games.