Best way to natively tag photos on OS X

Highlight the photo you want to tag in Finder, then press 'Command' and 'i' on the keyboard. At the top of the dialogue box you should see 'Spotlight comments' (you may need to hit the disclosure triangle to reveal the field). You can add tags in there.

To find a photo with a given tag, just hit the Spotlight icon and type in the tag word.

Trouble is, Spotlight comments are restricted to Macs (they use .ds_store files) and the Windows commenting system you mention in your first post is restricted to Windows. Neither actually "natively tags" the file in an environment-neutral way.

However, EXIF data is actually embedded in image files. You can edit a file's EXIF data from within iPhoto if you want. I do understand what you mean about longevity, but I would honestly prefer to use something like iPhoto's tagging features, which are much more flexible, and make it really easy to tag and smart-group pictures into sets. You don't have to use any of iPhoto's editing features - it does very well on organization alone. If you absolutely don't want to touch your existing folder structure, get Aperture instead, which allows you to keep the organization you've done.

Matt

Keywords can be whatever you like, and you can apply multiple keywords to the same image (and quickly apply the same keyword/s to multiple images). You can create a smart album (basically a search folder) to say "show me all the images tagged with holiday + sun + dad, for example).

Matt

If you absolutely don't want to touch your existing folder structure, get Aperture instead, which allows you to keep the organization you've done.

As does iPhoto since, oh, 2006.

Which are you asking about? iPhoto or Aperture?