Oils in general are the most common example of nonpolar liquids, including regular vegetable oil and related products like gasoline. Nonpolar liquids rarely mix with polar liquids, which is why oil and water stay separated when put in the same container.
A substance is polar or nonpolar depending on the orientation of its valence electrons. When the electrons are distributed asymmetrically, the molecules have a localized negative charge in one area. This area is called a dipole. A nonpolar substance consists of molecules with no dipoles. As a general rule, polar substances will interact with other polar substances but not nonpolar substances.