Operator is an interesting extension for Firefox that enables the browser to process select microformats.
Operator lets you combine pieces of information on Web sites with applications in ways that are useful. For instance, Flickr + Google Maps, Upcoming.org + Google Calendar, Yahoo! Local + your address book, and many more possibilities and permutations. All of these scenarios are possible due to Microformats, an emerging standard for injecting semantics into HTML.
For an example of how this works, check out the screenshot of Yahoo! Local, which supports hcard. The search results page returns the normal search results page, but because the results have microformat encoded data the Operator extension picks them up and enables the dropdown on the menubar which makes it possible for me to pass the data to the “handler” designated for that format, in this case my address book app.
While the vcard and hcard formats are interesting, I actually found the rel-tag capabilities more useful because on any webpage I could drop down del.icio.us, flickr. and technorati menus prepopulated with tag data.
This extension points to a future scenario where the browser is doing a lot more heavy lifting to move data from one web service to another. I would think it would be really useful to be able to take a browser window with a number of tabs open and move data from one to another based on the formats that are generated and capable of being consumed.