Useful Free Plugins

Essentials, PermissionEX, World Edit.
Those are basically all you'll really need for a basic server.
If you're going to put in a lot of effort, you'll want to look into different group managing plugins and things like that.
I like to download hundreds of things from the Fun section on a lot of these forums for like Spigot and CraftBukkit. Those are usually what will bring in players and make the server more fun overall