A Heads Up Display (HUD) shows information reflected off the windshield, such as the speed and any warnings. More advanced systems may include navigation data as well. This allows you to read the information while keeping your eyes focused on the road.
The best-designed systems set the focal range of the display to be infinity so you don’t have to re-focus to see the information, but this rare in today’s systems.
GM, BMW, Nissan, and a few others have experimented with HUD technology and have offered various versions on select models in the past. Often the systems difficult to use in all light conditions, partly due to attempts to keep the costs somewhat reasonable (although still expensive). On the 2013 BMW M5, the Heads Up display is a $1300 option.
There are third-party products that can be attached to the car, typically in the $100-150 range that display limited information, such as speed.