What are /r/minecraftmaps' favorite Products & Services?

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You actually have many options. The first and probably easiest is World Edit. That link is to the bukkit plugin, but there's also a Forge version if you don't want to mess with bukkit. There should be a link to the Forge version on that page.

Alternatively, you could also use MCEdit which is arguably more powerful but definitely a harder learning curve. This is an app you run on your local computer. You point it to the world's level.DAT, and it allows you to...well edit the world.

It’s not exactly a map, but I created a plugin for Spigot which generates SkyGrid worlds.

https://www.spigotmc.org/resources/skygrid-1-13-1-16-4.66996/

You would need to run a Spigot server to play it, but I intend to keep it updated to future MC versions too.