Yahoo! BrowserPlus

Yahoo! BrowserPlus is software that extends the capabilities of web browsers to make rich web experiences possible. Websites can use the software to support behaviors like drag and drop from the desktop, easier file uploads, acquisition of feeds and information, and native desktop notifications.

The site is showing very cool demos right now including dragging and dropping photos from the desktop to a web browser, live image manipulation, and live refreshing IRC chatrooms in the browser. One of the nice things about how this is implemented is that when you've arrived at a site requiring BrowserPlus, the software is installed in your browser and does not require a restart.