GroupManager is a simple and reliable open source permission management plugin for the Spigot/Bukkit based Minecraft servers, it runs "out of the box" without configuration, but is highly configurable. It's licensed under the GNU General Public License v3.0.

PermTrigger is a command trigger plug-in. It allows you to add sets of commands to run when a player has a permission granted or taken away. It's licensed under the GNU General Public License v3.0.

Regen is an explosion regeneration plug-in. It replaces blown up blocks to keep worlds looking pretty. It's licensed under the GNU General Public License v3.0.