A keyboard and mouse are probably the most widely used input devices. Depending on your relationship with the device, you could consider things such as a network card as an input device (i.e. when you are communicating with a server)
They are many, input devices basically allow you to enter data to the computer, eg, keyboard, mouse, web cam, DVD drives, joy stick, et,c...some double as both input and output.
This would depend on what type of computer you are discussing... - For a personal computer (Desktop) or a Laptop this would be a keyboard followed closely by a mouse or touchpad. - For a tablet this...