Peripheral membrane proteins are proteins that loosely and temporarily adhere to the plasma membrane using a combination of electrostatic, hydrophobic, and other non-covalent interactions. They may either attach to the lipid bilayer or to integral proteins. 

Cytochrome C, a protein involved in electron transfer for the electron transport chain is one example of peripheral membrane proteins. Other examples include high potential iron protein, cupredoxins, adrenodoxin reductase and some flavoproteins among others.