Rainmeter is a free desktop customization tool for Windows that allows users to create and display customizable skins on their desktop. These skins can show various information, such as system performance, weather updates, and calendar events, enhancing the overall user experience. Users can download pre-made skins or create their own using a simple scripting language. Rainmeter supports a wide range of plugins and features, making it a versatile choice for those looking to personalize their desktop environment while keeping important information easily accessible.