Previewers
Previewers are custom Javascript code to visualize information about datasets and files (collection support is experimental). Usually those are used to provide a preview of a file, when the file is big, but they can be used to visualize more interesting aspects of a resource. For example, the GIS previewers enable overlaying geospatial data on a interactive map in the browser.
Previewer can work together with extractors and external services.
Here is a list of previewer embedded with the core source.