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Membrane Contact Keyswitches
The membrane contact keyswitch is a variant of the rubber dome design and works in a rather similar way. The basic mechanism is the same: contact pairs on a circuit board, and rubberized boots or "dimples" with a carbon button underneath. Press down the key and the rubber deforms, the carbon touches the contacts and a keystroke is sensed.
The big difference here is that individual keycaps and plastic plungers are replaced with a thin membrane that fits over the rubber domes. There may not even be separate rubber domes, just molded "dimples" for each key, the carbon buttons in each dimple. The user presses directly on the membrane to collapse the domes and create contact with the printed circuit board. Travel is very small with this design, since there is no keycap and no plunger.
It's debatable as to whether such a design is really a "keyboard" any more, in fact. At any rate, this technology is not typically used for main PC keyboards, because the lack of proper keycaps and very short travel makes touch typing virtually impossible. A great deal of force in the middle of the key is usually required for a keypress to register, and so typing speed is reduced.
This technology has some advantages over the others, however. One is that it is quite cheap to make, because you eliminate a large percentage of the parts from a typical keyboard design. Another is that the solid membrane seals dirt and contamination out of the internal workings of the keyboard. For this reason, this technology is sometimes used in industrial applications. It is also often found in PC peripherals, and other devices that require keystrokes but not full-fledged typing as with a PC.
Next: Capacitive Keyswitches