MIT researchers used micro-manufacturing technologies to build ion thrusters smaller than a penny that could propel CubeSats in space. Smaller than a milk carton, CubSats are relatively inexpensive and several can be delivered into orbit on a single rocket. From MIT News:
Engineering propulsion systems for small satellites could solve the problem of space junk: CubeSats could propel down to lower orbits to burn up, or even act as galactic garbage collectors, pulling retired satellites down to degrade in Earth's atmosphere. However, traditional propulsion systems have proved too bulky for nano satellites, leaving little space on the vessels for electronics and communication equipment.
In contrast, (aeronautics/astronautics professor Paulo) Lozano's micro thruster design adds little to a satellite's overall weight. The microchip is composed of several layers of porous metal, the top layer of which is textured with 500 evenly spaced metallic tips. The bottom of the chip contains a small reservoir of liquid — a "liquid plasma" of free-floating ions that is key to the operation of the device.