So, Heat metal is one of the most fun, funny, and interesting spells in the Druid (and upcoming Bard) repertoire. It's use is sometimes referred to as "Cook and Book" - coined by Zee Bashew in the Animated Spellbook (see below).
For those that don't know; Heat metal works on anyone wearing armor or using metal weapons, it causes an initial 2D8 fire damage and last for a full minute (10 rounds) - where you can use a Bonus Action to re-apply the 2D8 damage. It ALSO causes disadvantage for the victim on their attack rolls. It is VERY useful against the metal Constructs in the Arcane Tower, the "Paladins" of Tyr in the Tollhouse, the Githyanki and the Duergar in the Decrepit Village.
It is Concentration based - hence the BOOK part since initial range is 60 feet but range is unlimited AFTER you cast it on someone - you can run like hell so no one can break your concentrations.
However, with the right gear you can do 4D8 per turn instead of just 2D8. If you grab the Circlet Of Fire from the Horde of Dror Razglin or from taking the rescue the Grand Duke quest at Waukeen's rest it's special ability will grant you a bonus action when you use the Heat metal ability as a bonus action on round two, which then allows you to use the Heat metal Bonus action ability again in the same turn! 4D8 damage per turn baby!
As has also been pointed out the Circlet also proc's off hitting someone with the Flaming Scimitar - allowing you to attack again with it if it is held in the off-hand - making the Circlet of Fire an incredibly powerful piece of equipment for the Druid class.
Credit to my friend Krombopulous Skunk for discovering the Heat Metal use. And now please enjoy this Animated Spellbook on Heat Metal or as we like to call it - "Cook and Book"