ProtectionAreas (built-in)

[DiamondToaster]

The problem with that is creepers can venture near the bounding box that houses the area, but not enter it. If close enough and still existant, the creeper could blow up blocks near the edge. As xoft said, currently there is no way of determining exactly which blocks will be affected by the blast. A second problem with that is if a plugin were to use a hook that checks a mob's position everytime it moves/per tick and there are hundreds of mobs, there would be a relatively large overhead. Since the plugins are implemented in Lua, it is significantly slower than using native code, thus creating an even larger overhead (unless LuaJIT is being used).