Excess cooking oil can be reused to cook your next meal! After letting the oil cool, strain it through a fine mesh strainer or coffee filter. Next, place it into an airtight container and store it out of direct light. Check the oil for quality before using it each time. For best results, cook with a combination of new and used oil.
Cooking oils can serve as a natural lubricant and alternative to WD-40. While great for preventing rust, WD-40 is designed for "water displacement", not lubrication. Try used cooking oil on your tools for better results. Be aware of any heat generated by the tool, however. Some oils may smoke or burn at relatively low temperatures. Oils that tolerate higher temperatures well include grapeseed, peanut, sunflower, safflower, soybean, olive, and canola oil.
Cooking oils can be used as a natural sealant for wood. If you don't have enough used oil for your project, you can supplement it with linseed/flaxseed oil from the grocery or hardware store. Always be sure to use food-safe oils for projects such as dining tables, cutting boards, etc.
Soap is made through the process of saponification, in which an oil is combined with lye. This process is simple enough that it can be done at home with your used oil. Remember to always use caution and take appropriate safety measures when working with lye.
Oil can be added to your compost pile in small quantities. Use moderation, as oil slows the composting process.
Oil when burned emits harmful greenhouse gasses.