I just discovered today that Flickr is now using ExifTool to extract meta information for its database. Although they don't acknowledge this, it is obvious from the ExifTool-specific formatting of some values. It looks like they made the change around the end of December 2008 because images uploaded after this all seem to use ExifTool. Cool.
Below is the the text displayed by Flickr when you click on
"More properties" for a recently uploaded image from a
Canon PowerShot camera. If you use ExifTool a lot, you should
recognize that the values correspond exactly to ExifTool's
(sometimes quirky) formatting: