The best free alternative to Bukkit is Glowstone, which is also Open Source. If that doesn't suit you, our users have ranked eight alternatives to Bukkit and eight of them is free so hopefully you can find a suitable replacement. Other interesting free alternatives to Bukkit are PaperMC, Void Proxy, Pumpkin and LeafMC.
Glowstone is a lightweight, from scratch, open source Minecraft server written in Java that supports plugins written for the Spigot (and Bukkit) API.
Paper is a fork of the Spigot server implementation (which is itself a fork of CraftBukkit). Paper strives to bring improved performance, more features, and more APIs for developers to build awesome plugins with.
Void is an open-source Minecraft proxy - an alternative to Velocity or BungeeCord - designed specifically for modded servers.
Minecraft server built entirely in Rust, offering a fast, efficient, and customizable experience. It prioritizes performance and player enjoyment while adhering to the core mechanics of the game.
Simply put, Spigot is it. Spigot is the most widely-used modded Minecraft server software in the world. It powers many of the top Minecraft server networks around to ensure they can cope with their huge player base and ensure the satisfaction of their players.
A custom Minecraft compatible game server written in C++. It's a fast, lightweight, and awesome replacement for Vanilla.
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Good alternative for bukkit. Spigot have bukkit plugin support.