Leaflet is a simple javascript framework for building web-mapping applications. In thier words:

Leaflet is a modern, lightweight open-source JavaScript library for mobile-friendly interactive maps. It is developed by CloudMade to form the core of its next generation JavaScript API. Weighing just about 21kb of gzipped JS code, it still has all the features you will ever need for your web mapping needs, while providing a fast, smooth, pleasant user experience.

It is built from the ground up to work efficiently and smoothly on both desktop and mobile platforms like iOS and Android, utilizing cutting-edge technologies included in HTML5 and CSS3. The focus is on usability, performance, small size, A-grade browser support, flexibility and an easy-to-use API. The OOP-based code of the library is designed to be modular, extensible and very easy to understand.

When you're choosing a javascript framework to build your web-mapping application, you have a myriad of options. Why choose Leaflet? Well this is why I liked it: