Mozilla Prism is a project created by the Mozilla Foundation (Firefox and Thunderbird creators). Prism is dedicated to bridge Web apps with desktop apps. The project is based on Webrunner which moved to Mozilla code repository and took the name Prism.
The first thing you may think is that “We have already Microsoft Silverlight and Adobe AIR for that kind of stuff”. Thats true, but first of all Prism is open-source, which means that developers around the world can add features or help to the development and fix security issues. Secondly Prism is not aimed to replace the Web. Its just acts like a bridge (like a mirror) between web apps and your desktop. Lets use an an example to understand how Prism works. Assuming that Gmail is your favorite web app. Prism lets you split Gmail out of your browser window and run it directly from your desktop on its own window.
When the final version will be released, Prism will probably be integrated with Firefox and you will not even have to download and install it. A button in Firefox that will let you with one-click to add the web app you like on your desktop is also in plans. You can download the current beta version of Prism (Microsoft Windows version only) from here.
If that project succeeds, it will be a great step on improving the web experience and bring the web “everywhere”. Its big advantage agains Silverlight and Adobe AIR is that it can bring any web app in your desktop and not only the apps that are created using specific frameworks. As the project is open-source, you can participate in the development. Most info on that, here.