It's used by all the browsers on Windows these days. They all create a 'Zone.Identifier' stream when a file is downloaded to mark is downloaded. It's content's is what triggers the "You downloaded this file! It's Evil!' warning in Windows.

To be fair, it's not used by a ton of things, since it requires NTFS, disappears when files are moved to different filesystems, and various things that read and write files destroy them if they're not careful, not to mention actually enumerating the streams is tricky, last I checked.