Thermal grease is a substance that increases the thermal conductivity of a thermal interface, such as a computer chip, by compensating for the irregular surfaces of its components. In the electronics and computer industries, it is used to help a heatsink draw heat away from a semiconductor component, such as an integrated circuit or transistor. If something goes wrong with your heatsink and you're out of thermal grease to repair it, the best thing to do is to get by without your computer until you can get to the electronics store. If you can't wait, though, other temporary, though less safe, options are available.
Dielectric silicone is used as a heat conductor for HEI ignition and is designed to last indefinitely in high heat conditions in automotive applications. It's not electrically conductive, but it can last anywhere from several months to many years. It's not a very common household item, but it's fairly cheap and can be purchased at any automotive store.
High Temperature Grease
Used in automotive applications as a lubricant, high temperature grease is meant to endure high temperatures without thinning out. As a result, although it's not effective for thermal conductivity, it lasts for a long time. It is not a common household item, but it can be purchased at any automotive store.
Liquid Gel Deodorant
Found in many stores, liquid gel deodorant is easily accessible. However, it is designed to evaporate quickly and will provide good thermal conductivity only until it dries. The longest period for which it will stay moist is one day in an enclosed environment, depending on the conditions.
Toothpaste is readily available, and most homes have a tube lying around. It cools well. However, its lifespan is short---only approximately 12 hours because it becomes dry and crusty quickly.
Lip balm is another cheap, easily accessible product. Lip balm is paraffin-based, so when it gets hot, it tends to get runny. It also tends to become solid over time. Therefore, its lifespan used in this application is less than one day.
Antiseptic gel is less common than toothpaste, but it can be purchased at any store and is usually found in first aid kits. It cools well, but it has a short lifespan because it is water-based and will quickly evaporate.
Hair Molding Clay
Even though no guy would admit to owning a tube of hair moulding clay, it dries slowly and can be a long-term solution (up to three days). It is water-based and contains paraffin wax, so it's not a very good thermal conductor.
Another common household product, aluminium foil is surprisingly effective and works as well as any of the liquid options. It never dries out, so it lasts indefinitely. Unfortunately, however, it doesn't cool effectively and is very electrically conductive.