File Sheriff (Mac OS X) is a good general viewer and editor with limited metadata support.
Finder (Mac OS X) displays limited metadata and allows editing of Finder Info tags.
mdls (Mac OS X) is the built-in command line tool to display a listing of metadata attributes.
MetaGrove (Mac OS X, Windows) is an extensive commercial suite of tools to support Adobe XMP metadata, particularly in CS3 and related applications, and digital asset management; it consists of MetaGrove Developer ($499) to edit and manage XMP metadata, plus plug-ins for CS apps prior to CS3.
Punakea (Mac OS X) supports metadata tag creation and editing.
TagBot (Mac OS X) tags any Mac files with metadata tags.
Adobe Photoshop CS3.
Adobe Photoshop Lightroom.
Aperture (Mac OS X), Apple.
GraphicConverter (Mac OS X) displays EXIF and XMP metadata and allows editing of IPTC data.
iPhoto (Mac OS X), Apple.
IPTCViewer (Mac OS X) displays metadata in InDesign and automatically adds byline and caption text; aimed mainly at photo-agency work.
J2SEditor and its relatives (Mac OS X) are metadata editors and more for IPTC data in images.
METAmachine (Mac OS X) is a metadata editor for XMP formatted metadata in TIFF, JPEG, PSD and DNG images to IPTC standards.
Reveal (Mac OS X, Windows, Linux) is a metadata viewer with limited editing functions for EXIF metadata in most image types.
AtomicParsley (Mac OS X, Windows, Linux and Unix) is an open source command-line tool for editing MPEG-4 metadata, and is used by MetaX and Lostify.
Final Cut products (Mac OS X), Apple.
iMovie (Mac OS X), Apple.
Lostify (Mac OS X) is a metadata editor for MPEG-4 files aimed at iTunes movies.
Metadata Hootenanny (Mac OS X) supports viewing and editing of QuickTime metadata; it is aimed at QuickTime movies.
MetaX (Mac OS X) is a metadata editor for MPEG-4 files, and includes Amazon searches for movie and TV show artwork; it is aimed mainly at movie collections.
QuickTime (Mac OS X, Windows) particularly in the paid-for Pro version.
BWAV Reader (Mac OS X) is a metadata reader for broadcast WAV (BWAV) files, whilst
BWAV Writer (Mac OS X) is a metadata editor for broadcast WAV (BWAV) files.
iTunes (Mac OS X, Windows) from Apple, and most other audio players.
remetacator (Mac OS X) is a metadata editor for broadcast Wave files, notably of BWF metadata for Avid workstations.
Tag (Mac OS X) is a metadata editor for FLAC, Ogg Vorbis, Monkey's Audio and WavPack files.
MailTags (Mac OS X, Mail) is a metadata editor for (Apple) Mail email.
BSP (Mac OS X) adds custom Spotlight categories for search results.
Desk Lamp (Mac OS X) extends Spotlight search facilities, using Spotlight metadata.
DPX and Cineon Spotlight Plugin (Mac OS X) makes DPX and Cineon metadata available as Spotlight (etc.) metadata.
McNucle (Mac OS X) is a client-server metadata system in public beta; it is aimed at sharing metadata features across networks and the Internet.
Meta (Mac OS X) offers an enhanced Spotlight search, using Spotlight metadata.
Punakea (Mac OS X) has various enhancements to Spotlight, particularly with custom tags.
Spotlaser (Mac OS X) offers an enhanced Spotlight search, using Spotlight metadata.
Spotlight file system for MacFUSE (Mac OS X) turns Spotlight into a ‘proper’ file system, within a userland application.
Spotlight Indexer (Mac OS X) provides a GUI front end to mdimport, Spotlight’s indexing tool, that allows you to extend Spotlight coverage to other locations, e.g. hidden folders.
SpotMeta (Mac OS X) adds metadata tags to extend Spotlight searching.
ApertureToFileMaker (Mac OS X, Aperture, FileMaker) exports Aperture metadata to FileMaker databases.
Metadata Export for iPhoto (Mac OS X, iPhoto 6) is a plug-in for iPhoto that allows you to export photo metadata in a range of formats, including XHTML and XML.
Code Collector Pro (Mac OS X) is a metadata-based organiser for source code snippets.
EagleFiler (Mac OS X) is an email and document organiser/archiver, accessing metadata, that works with MailTags.
Eureka! (Mac OS X, Mac OS 9, Windows, Linux) is a document organiser/archiver/searcher, accessing metadata only from its own database, not Spotlight.
iGTD (Mac OS X) is a 'Getting Things Done' styled organiser, that accesses metadata and works with MailTags.
MailSteward series (Mac OS X) are email organisers/archivesr, that access metadata and work with MailTags.
MetaSync (Mac OS X) synchronises iView MediaPro metadata.
Minim (Mac OS X) is a metadata-based organiser for compositions and music.
(Mac OS X) an excellent article looking at problems that different command line tools have in handling metadata safely.
(Mac OS X) an excellent study of the problems of handling metadata, particularly in a Mac OS X Server environment.
(Mac OS X) a suite of command line tools for working with xattrs.
IPTC includes photo- and press-related standards
Mac OS X 10.4
RDF a vast guide to the RDF parent
RDFa for web interoperability
Spotlight developer docs