Bukkit is GPL. It states it explicitly here: https://github.com/Bukkit/Bukkit/blob/master/LICENCE.txt The GPL requires all derivitive works (anything that links into bukkit code, event listeners, api, even extending JavaPlugin) must be GPL: Note the red section. It's cut and dry. All plugins and any derivative works (Pail, etc) must be GPL. VERY, VERY few are. In fairness, none of my plugins are GPL, so I too am breaking the US copyright law. Now, I believe this is the point where someone steps in and says "Bukkit's license is invalid, they are reviewing it". If true, that means the Bukkit Team actually hold an absolute copyright on the bukkit project (much like the copyrights on music and movies) and anyone who uses bukkit code (plugins, etc) are STILL breaking copyright law, since none of us have written permission to use bukkit code. For the tl;dr: Bukkit is GPL, so 99% of plugins are illegal, and if Bukkit is not GPL, 100% of plugins are illegal. Why do I bring this up? Besides the obvious, the Spout team is in the process of writing a client side API. We're hoping to borrow existing bukkit code (Note, Biome, Inventory, ItemStack, etc) instead of rewriting it from scratch. For the moment, we are breaking copyright by doing this. We'd like to hear confirmation from bukkit.