Google has recently released five mini-games for Glass. The games include Tennis, Balance, Clay Shooter, Matcher, and Shape Splitter. These will be available using the "Ok Glass, Play a Game" command, but perhaps key here, Google has said they released these games in an effort to inspire developers.
By inspiring developers, Google is also showing off some of the capabilities of Glass. These games take advantage of the sensors found within Glass and also make use of voice commands. These games were intentionally developed to be simple. In fact, Google mentions how each of them are "visually simple and straightforward to play."
Further chatter here touches on how they are easy to pick up when you have some free time, and also just as quick to drop when you need to get back to doing something a bit more productive. Simply put, these are built to show developers, and likely users, some of what can be done with Glass.
As you'll see in the video, you have the voice commands as well as head movements and hand gestures. The real question may be in terms of how someone would look when playing these in public. Those worried about getting strange looks while wearing Glass may be in for some additional looks when playing Clay Shooter which has you using commands such as pull and bang, bang.
That aside though, Google also talked a bit about what the games were built with. One common theme here was the Min3D library and OpenGL. That was involved with Tennis, Clay Shooter, and Matcher. In addition, Matcher also makes use of the Photosphere camera mode to map surrounding cards. Lastly, Balance is described as being a "robust physics simulation" that was built using Box2D and AndEngine.