This is just a quick (and non-exhaustive) list of tools that I use to tag various system objects:
- To tag files: Fresh;
- To tag photos (in iPhoto): Keyword Manager;
- To geotag photos: HoudahGeo;
- To tag email messages (in Mail.app): MailTags;
- To tag bookmarks: delicious.com.
Stay tuned, I will review these and other applications in the coming weeks.