Lily is a Firefox add-on that allows users to create rich, complex applications in a web browser. With Lily it's possible to make programs that mashup the web or the desktop, visualize and animate data, modify webpages, play music, or connect to world outside the computer. Lily programs can be shared with other Lily users as text files or run by anyone as Firefox add-ons or standalone XULrunner applications. Lily runs on Mac, Windows or Linux, just like Firefox.
There are over 180 externals objects built into Lily: web service modules for APIs like Twitter, Amazon, Flickr, Wikipedia, Yahoo; UI modules that wrap web components from YUI, JQuery and Google Maps as well as the browser's built-in UI elements; modules that offer access to the network, SQLite storage, TCP sockets or the file system; modules to interact with the browser; modules to send and receive Open Sound Control messages or talk to the Arduino physical computing board; graphics modules that encapsulate the browser's SVG functionality and multimedia modules for playing sound and video.
Lily is free and open source, released under the MIT license.