Explorers are great medics.
Iron Clads are good at defending fishing and coastal cities with 1-2 tiles of access to the sea to prevent direct drops and guarding tight inland ocean routes, just have trouble killing anything that doesn't come to them, making them useless for defending large areas. ( altho I believe the circumnavigate bonus gives them 3 movement)
ironclads also have the bonus of not needing astronomy to use, so you can defend fishing without getting sea techs
Chariots unuseful? your nuts.
Machine guns can defend a city without worry of them falling until tanks or massive cavalry/artiliery.
I haven't found a good use for guided missles since I don't think they can kill anything, but can make the diffience in a naval battle when fighting 40 vs 40 or 40 vs 33, course if you attack first the collateral damage will do about the same.
Horse archers can use flanking and harm any incoming catapults in stacks and a good counter to longbows and crossbows.
If you play online warriors can save your life, especially when your sacrificing 2-3 to take down a chariot, or producing one a turn to keep a lone axemen from taking a city until stronger troops arrive, or guarding a worker in a forest.
I can't really think of any unit that doesn't have a purpose, but others are more situational then others, like the ironclad when movement isn't a issue.