Sodium carbonate, or washing soda, is a useful general purpose cleaning product. Sometimes called soda ash, it's technically a salt made from carbonic acid, and can help you unclog drains, clean decks, remove grease from cooking implements, keep pests off plants, and coax out pesky stains in laundry, among other things.
For fabric application, you can either pre-soak the items you want to clean before you launder them, or you can add it to your homemade laundry detergent for regular loads. The chemical softens water by bonding to minerals, allowing detergents to move more freely and clean more thoroughly.
For garden pest control, dissolve half a cup of washing soda into two gallons of water and spray or rub the mix onto the plants you want to defend.
To make your own sodium carbonate, heat baking soda in a baking dish for two hours at 400 degrees Fahrenheit, stirring and leveling once halfway through. Application to skin can cause irritation or burns, so keep kids away and avoid contacting large quantities directly.