Chrome extensions are still sandboxed in many ways, which restricts their potential. By using a cross-platform Java program, opened by Java Web Start, to listen to requests on a port on the localhost, the Chrome extension can gain access to the relatively unrestricted Java API.
For example, I use this framework to recursively list Dropbox directories and allow users to right click on Chrome items to download them straight into one of those folders. If the file is saved in the Public folder the public URL is copied to the clipboard.
As another example of this framework, right clicking on links can also show a "Open in Firefox" option.
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