Xattr

Xattr tags are "eXtended Attribute" tags, which is a feature of system 10.4 or later. They're like "spotlight comments" in that they are a type of meta tagging facility built into the file system when seen thru the lense of the Mac OS. (they also work on FAT32 volumes with the "._" prefixed sidecar file). For the most fantastic explanation ever, see the Ars Techica article on it.

Anyway I use xattr tags currently on all QuickTime compatible files that do not have the ".mov" extension.