The voltage difference created by the thermal motions of electrons. This is pretty small, but it increases with temperature (duh) and resistance (not duh). This is only rarely a major contribution to the total noise in a circuit.
An equation giving the strength of the noise is given so: <V^2> = 4 R kT δf
Note that this falls afoul of the ultraviolet catastrophe, and is subject to its solution: the electrons will at some point begin to have too little energy to emit phonons of the appropriate frequency. This is represented by an attenuation factor of e^(-hν/kT).