One I use a lot: mdls.
It's ls for metadata. Very helpful for getting quick and scripted access to the date/time when and the latitude/longitude where a photograph was taken.
Also, the say command is a lot more versatile than it seems.
Combined with the ability to save the speech to an audio file, my wife uses it as the disc jockey for her little hobby AM radio station. It introduces the song and does little station IDs and such.
You can customize it with dozens of dozens of voices, some in very high quality.
When a song from her Japanese playlists comes on, it switches to one of the Japanese voices. I don't speak Japanese, so I don't know if it actually translates the DJ words into Japanese, but it sounds pretty close to my untrained ears.