This is one of the most essential tools in a Thai kitchen. Used to pound chilli, garlic and other spices for most of the Thai recipes, you will learn very fast that a mortar is great. And the bigger the better. I once bought a mortar here in Europe which was way too small. Now I know of large, wooden mortars (like in the picture) and the ones made of stone (granite). Really heavy to lift but for pounding very stable. If you are about to cook a Thai or other asian dish, be sure to get your hands on a large, heavy mortar!
Image taken on a market in Thailand.