Towny is a plugin that lets players create towns to claim 16x16 chunks of land. Towns protect your land from griefing. You can also make nations to form groups of towns and ally with other nations. Towny has many related plugins, which are listed below.
Related: TownyCultures, TownyResources, SiegeWar, Quarters

QualityArmory (Guns)
Fairly straightforward, this plugin adds guns that you can craft with the command /qualityarmory craft. It's not regular crafting, you spend a certain number of items to get the guns, like iron and redstone.

Economy: DivinityEconomy and player based economy plugins
QuickShop-Hikari, Player Auctions, AxTrade, Rock Paper Scissors, and Lottery

We have many other plugins which are not mentioned here but are very useful, like ViaVersion and Dynmap