NPC Plugins allow you to place entities around the map which can be interacted with. Often used to execute commands on-click or to execute dialogue.
Citizens is the most popular and widely supported NPC plugin in the market. It is purchasable on Spigot and open source as well. Free downloads can be found on it’s Jenkins server. Not recommended for servers with high player counts as it can eat a lot of ticks.
ZNPCs is another popular NPC plugin. Although it is better than Citizens from a performance perspective, it is quite janky and unpleasant to setup.
ZNPCsPlus delivers great performance while also being easy to setup & edit. Recommended as long as you don’t mind the lack of support in other plugins for it, V2 can be compiled from the Github page.