WD-40, Unilease, GT85, Kroil and CRC 5-56 are well-known brands of penetrating oil.
Penetrating oil is very low viscosity oil which can be used to free rusted mechanical parts (such as nuts and bolts) so that they can be removed as it can penetrate into the narrow space between the threads of two parts.
Penetrating oil can also be used as general purpose lubricant, or to clean or prevent corrosion.