An immersion blender is a great tool. No kitchen should be without one for instant pureeing of soups and sauces right in the pan.
A tablespoon of really fine butter, stirred in at the end of making a dish, can mean the difference between an everyday sauce, soup or risotto and something extraordinary.
A well-balanced knife is indispensable. You don't have to go to cooking school to learn how to chop; to truly master knife skills, practice and practice some more. Invest in an eight- to nine-inch carbon-steel knife and a sharpening stone.