Xattr

Xattr tags are "eXtended ATTRibute" tags, which is a feature of MacOS 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, and also on zip files so i do not need to open the zip to get at the tagging info. Here is what my xattr looks like:

[oxford:kj/media/bin] davec% xattr -l "Rave On.bin"
Rave On.bin
        com.meta.albm   Unknown Album
        com.meta.arts   Buddy Holly
        com.meta.crdt   2006
        com.meta.genr   Rock
        com.meta.name   Rave On
        com.meta.soft   kJams Pro 1.0d45r21
        com.meta.trak   1
[oxford:kj/media/bin] davec%

Note: It's a very simple format, I'm just using QuickTime “Common Key Format” tag names (from Movies.h) as the keys (prepended with "com.meta."). I've added a few others:

							
/*
  kQTMetaDataCommonKeyAuthor			= 'auth',
  kQTMetaDataCommonKeyComment			= 'cmmt',
  kQTMetaDataCommonKeyCopyright			= 'cprt',
  kQTMetaDataCommonKeyDirector			= 'dtor',
  kQTMetaDataCommonKeyDisplayName		= 'name',
  kQTMetaDataCommonKeyInformation		= 'info',
  kQTMetaDataCommonKeyKeywords			= 'keyw',
  kQTMetaDataCommonKeyProducer			= 'prod',
  kQTMetaDataCommonKeyAlbum			= 'albm',
  kQTMetaDataCommonKeyArtist			= 'arts',
  kQTMetaDataCommonKeyArtwork			= 'artw',
  kQTMetaDataCommonKeyChapterName		= 'chap',
  kQTMetaDataCommonKeyComposer			= 'comp',
  kQTMetaDataCommonKeyDescription		= 'desc',
  kQTMetaDataCommonKeyGenre			= 'genr',
  kQTMetaDataCommonKeyOriginalFormat		= 'orif',
  kQTMetaDataCommonKeyOriginalSource		= 'oris',
  kQTMetaDataCommonKeyPerformers		= 'perf',
  kQTMetaDataCommonKeySoftware			= 'soft',
  kQTMetaDataCommonKeyWriter			= 'wrtr'
*/
							
enum {
	CM_Key_TESTAMUNDO		= 'tmdo', 	//	bool
	
	CM_Key_NONE			= 'xxxx',
	CM_Key_ALL			= '****',
	CM_Key_NAME			= kQTMetaDataCommonKeyDisplayName,	//	'name',
	CM_Key_ARTIST			= kQTMetaDataCommonKeyArtist,		//	'arts',
	CM_Key_ALBUM			= kQTMetaDataCommonKeyAlbum,		//	'albm',
	CM_Key_GENRE			= kQTMetaDataCommonKeyGenre,		//	'genr',
	CM_Key_COMMENT			= kQTMetaDataCommonKeyComment,		//	'cmmt',
	CM_Key_KEYWORDS			= kQTMetaDataCommonKeyKeywords,		//	'keyw'
	CM_Key_SOFTWARE			= kQTMetaDataCommonKeySoftware,		//	'soft'
	CM_Key_WRITER			= kQTMetaDataCommonKeyWriter,		//	'wrtr'
	CM_Key_COPYRIGHT		= kQTMetaDataCommonKeyCopyright,	//	'cprt'

	CM_Key_SORT_NAME		= 'Snam',
	CM_Key_SORT_ARTIST		= 'Sart',
	CM_Key_SORT_ALBUM		= 'Salb',
	
	CM_Key_CUSTOM			= 'cust',
	CM_Key_CUSTOM_NUM		= 'cstN',
	
	CM_Key_SUB_GENRE		= 'SUBg',							//	string
	CM_Key_SwitchableAlbum_Dict	= 'LDct',	//	list of albums for popup menu, including default index value
	CM_Key_SwitchableSongs_Dict	= 'SDct',	//	list of songs for popup menu, including default index value
	CM_Key_PLI_INDEX		= 'piIx',	//	int, same as PLI_ID
	CM_Key_SORT_INDEX		= 'stIx',	//	int
	CM_Key_DUET			= 'Duet',	//	bool
	CM_Key_DUET_STR			= 'DueS',	//	STR

	CM_Key_BITRATE			= 'bRat',	//	string
	
	CM_Key_DOWNLOAD			= 'dl  ',	//	bool
	CM_Key_STREAM			= 'strm',	//	bool
	CM_Key_TIER			= 'Tier',	//	string
	
	CM_Key_TWIRLED			= 'Twrl',	//	bool
	CM_Key_LANGUAGE			= 'Lang',	//	string
	CM_Key_KEY			= 'Keyc',	//	string
	CM_Key_KEY_CHANGES		= 'KeCH',	//	string
	CM_Key_VOLUME			= 'volm',	//	float
	CM_Key_LABEL			= 'colr',	//	integer
	CM_Key_RATING			= 'rtng',	//	integer
	CM_Key_GRAPHIC_CHANNELS		= 'gChn',	//	integer

	CM_Key_COOP_THREAD		= 'Coop',	//	bool, must be extracted on a cooperative thread
	CM_Key_DEMUX_MPEG		= 'Muxd',	//	bool, must be demuxed

	CM_Key_PIPE_TYPE		= 'mPip',	//	int MetaPipeType
	CM_Key_PUSH_LEVEL		= 'mPsh',	//	int meta push level

	CM_Key_SONG_ID			= 'soID',	//	int
	CM_Key_SINGER_ID		= 'siID',	//	int
	CM_Key_PLAYLIST_ID		= 'plID',	//	int
	CM_Key_SINGER_PLI		= 'sPLI',	//	int

	CM_Key_TEMP_PLID		= 'sTid',	//	long	used for music stores and CDs

	CM_Key_FORMAT 			= 'Frmt',	//	string
	CM_Key_COPY_META_FLAGS		= 'cMfl',	//	int, based on CM_Key_FORMAT

	CM_Key_SOURCE			= 'SRCE',

	CM_Key_TRACK			= 'trak',	//	int
	CM_Key_YEAR			= 'cryr',	//	int (NOT date)
	CM_Key_ORIG_YEAR		= 'ORyr',	//	int (NOT date)

	CM_Key_START			= 'strt',	//	int sectors
	CM_Key_DURATION			= 'drtn',	//	int sectors
	CM_Key_PREVIEW_DURATION 	= 'pDrn',	//	int sectors
	
	CM_Key_SKIP_SILENCE_END 	= 'ssNd',	//	int subcodes (records)
	CM_Key_TRIM_START		= 'Tsrt',	//	int subcodes
	CM_Key_TRIM_END			= 'Tend',	//	int subcodes
	
	CM_Key_TIMES_SUNG		= 'tsng',	//	integer, times sung
	CM_Key_SIZE			= 'SIZE',	//	int
	CM_Key_DATE_LAST_SUNG		= 'Date',	//	date
	CM_Key_DATE_ADDED		= 'DAdd',	//	date
	CM_Key_PLAY_ME			= 'PLME',	//	bool
	CM_Key_PITCH			= 'PICH',	//	int
	CM_Key_CENTS			= 'cent',	//	float
	CM_Key_BPM			= 'bpm ',	//	int 
	CM_Key_TEMPO			= 'TMPO',	//	float 
	CM_Key_MISSING			= 'Msng', 	//	bool
	CM_Key_NO_CDG			= 'nCDG', 	//	bool
	CM_Key_IGNORE_RS_ERR		= 'igRS', 	//	bool ignore reed-solomon error correction

	CM_Key_SINGER			= 'SNGR', 	//	string
	CM_Key_SINGER_NICK		= 'nick', 	//	string
	CM_Key_SINGER_PASS		= 'Spas', 	//	string
	CM_Key_SINGER_SMS_ENABLE	= 'SmsB', 	//	bool
	CM_Key_SINGER_SMS_REGION	= 'SmsR', 	//	string
	CM_Key_SINGER_SMS_CARRIER	= 'SmsC', 	//	string
	CM_Key_SINGER_SMS_NUMBER	= 'SmsN', 	//	string
	CM_Key_BIO			= 'Bio ', 	//	string
	CM_Key_PRESENT			= 'PSNT', 	//	bool
	CM_Key_PLI_SORT_TO_TOP		= 'pStt', 	//	bool	//	not stored in PLI, but directly in Singer or PlayList
	CM_Key_SINGER_IS_KJ		= 'sgKJ', 	//	bool

	CM_Key_PRODUCER_DICT		= 'prod', 	//	dictinary
	CM_Key_MEDIA_STREAM		= 'meSt', 	//	dictinary

	CM_Key_SONGWRITER		= 'swrt',	//	string
	CM_Key_GENDER			= 'gndr',	//	string
	CM_Key_COMPOSER			= 'cpsr',	//	string
	CM_Key_PUBLISHER		= 'pbls',	//	string
	CM_Key_ORIGINAL_PUBLISHER 	= 'ORpb',	//	string
	CM_Key_ACTUAL_DURATION		= 'aDur',	//	string
	CM_Key_RENDITION		= 'rdtn',	//	string
	CM_Key_BACKING_VOCALS		= 'bVoc',	//	bool
	CM_Key_LEAD_VOCALS		= 'lVoc',	//	bool	 (center channel lead vocals)
	CM_Key_VOCAL_CHANNEL		= 'vChn',	//	integer
	CM_Key_COLOR_BACKGROUND		= 'cBak',	//	string
	CM_Key_EXPLICIT			= 'Expl',	//	bool, parental advisory lyrics

	CM_Key_SONG_SLIP		= 'Slip',	//	int
	CM_Key_CAVS_TYPE		= 'cavT', 	//	int
	CM_Key_CAVS_DATA		= 'cavD', 	//	cfdata

	CM_Key_FSRef_0_AUDIO		= 'Sfs0', 	//	CData::ValType_FILEREF
	CM_Key_FSRef_1_VIDEO		= 'Sfs1',
	CM_Key_FSRef_2_ZIP		= 'Sfs2',
	CM_Key_FSRef_3_AUD_CACHE	= 'Sfs3',
	CM_Key_FSRef_4_VID_CACHE	= 'Sfs4',
	CM_Key_FSRef_5_META		= 'Sfs5',
	CM_Key_FSRef_6_AUD_CD		= 'Sfs6',
	CM_Key_FSRef_7_VID_CD		= 'Sfs7',
	CM_Key_FSRef_8_UNPACK		= 'Sfs8',
	CM_Key_FSRef_9_TEMP_ZIP		= 'Sfs9',
	CM_Key_FSRef_10_PROJ		= 'Sf10',
	CM_Key_FSRef_11_LAUNCH		= 'Sf11',
	
	//	store related
	CM_Key_STORE_NAME		= 'StrN',	//	string
	CM_Key_STORE_NUM_SONGS		= 'SNmS',	//	long
	CM_Key_STORE_URL		= 'sURL',	//	string
	CM_Key_STORE_ID			= 'stor',	//	long

	CM_Key_SAMPLE			= 'Smpl',	//	string
	CM_Key_STREAMING_URL		= 'stmR',	//	string
	CM_Key_PRICE_USD		= 'pUSD',	//	string
	CM_Key_PRICE_USD_FREE		= 'USDf',
	CM_Key_PRICE_USD_STREAM		= 'USDs',
	CM_Key_PRICE_USD_BUY_STREAM	= 'USDb',
	CM_Key_PURCHASE_CODE		= 'pTRI',	//	string
	CM_Key_PURCHASED		= 'purc',	//	bool
	CM_Key_MUST_COMPLETE_PURCHASE	= 'CplP',	//	bool
	CM_Key_PLAY_AFTER_PURCHASE	= 'PlaP',	//	bool
	CM_Key_RECENT			= 'Rcnt',	//	bool
	CM_Key_STORE_SONG_ID		= 'SsID',	//	long
	CM_Key_STORE_CUSTOM_ID		= 'ScID',	//	long
	CM_Key_SONG_PAGE_URL		= 'spRL',	//	string
	
	CM_Key_SONG_NO_SYNCED_LYRICS	= 'sNSL',	//	bool
	
	CM_Key_SORT_INDEX_ROTATION	= 'rotn',
	CM_Key_ALPHA_LABEL		= 'ALFA'
};

For compatibility with 10.3 I'm using a standard plist sidecar file (XML) with the ".xattr" extension, which can be easily edited with any text editor, XML editor, or plist editor.