Final Fantasy 6 T-Edition is a huge patch to the game that fixes bugs, tweaks nearly every aspect of the game, and adds new features and content.
-Dozens of new music tracks
-New events
-New quests
-New bosses
-New enemies
-Re-balanced characters
-Bug fixes
-New features, like alternate costumes w/ alternate stats
-Lots of neat homages to other Square games
-Final Fantasy VI T-Edition EX - Once you've reached the Developer's Room, you can "save swap" to a ROM with the EX patch to fight even more super-bosses that had to be left out of the main T-Edition patch. The challenges never end!
The problem is that since it's in Japanese it's not that well-known outside of Japan, and making a translation patch out of it would be an enormous task.
More information can be found here at this site if you wish to take a look.
Here is the original site the patch is hosted on
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Final Fantasy VI (J).smc
CRC32: 45EF5AC8
MD5: 97BF78E916B80F47CF35EDF502BE34DC
SHA-1: 1C6A6DD90D3463F4532202199885A2C0E9A485F6
SHA-256: 7187333502021ADDC80CE27D5B0B4316DCAC9318871E1C3ED5ED474B10177B65