The Nanoship is (as its name implies) an unmanned microscopic ship, created by Professor Membrane. Primarily green and with microscopic cameras and sensors, along with an unknown type of propulsion, it is highly advanced in terms of human technology.
Its first appearance was during "NanoZIM", where Dib borrowed it to combat Zim, who had shrunk down to microscopic size in his own Irken Nanoship and invaded Dib's body. Professor Membrane's nanoship was armed with lasers and a grappling hook, and also had the ability to transform into a bipedal mech, much like Zim's Nanoship.
The Nanoship's controls are apparently similar to that of the layout for the Game Slave console that Dib's sister, Gaz, plays (not only are the typical D-pad and A and B buttons available, there is also an extra joystick). For this reason, Gaz was able to take control of the ship when Dib lost control of his own arms and proceeded to defeat Zim.
It is unknown if any more Human Nanoships exist, and if Dib still has it within him. However, it is presumed to have long since been lost in the local plumbing system.