Bukkit is described as 'Free, open-source, software that provides the means to extend the popular Minecraft multiplayer server' and is a minecraft server in the gaming software category. There are eight alternatives to Bukkit for a variety of platforms, including Mac, Windows, Linux, Minecraft Server and Minecraft apps. The best Bukkit alternative is Glowstone, which is both free and Open Source. Other great apps like Bukkit are PaperMC, Pumpkin, LeafMC and Spigot.
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.
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.
Void is an open-source Minecraft proxy - an alternative to Velocity or BungeeCord - designed specifically for modded servers.
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Good alternative for bukkit. Spigot have bukkit plugin support.