Out of all the free and/or storytelling systems I've purchased/looked at, I've played exactly none of them; Ironsworn was the first one that was a real gaming system. Delve expands that system and adds a focus on site exploration, but it's not just for dungeons; you can also use it for creating and exploring wilderness areas. It adds some delving specific moves and some elements that can be used in Irownsworn as well. It's not a necessary purchase to get the most from Ironsworn, but it is an excellent adition to it and an easy recommendation to purchase.
[5 of 5 Stars!]