I created this Minecraft server plugin using the Bukkit API to fix the "floating treetops" problem. When players chop a layer out of a tree's trunk, the rest of the tree falls down to rest on the remaining stump (any branches fall to the ground). This ensures no tree blocks will be left hovering in the air. Players who don't like it can disable it, unless that toggle is itself disabled by the server owner.
I stopped working on this project after Minecraft 1.10. Anyone may use my source code to continue the project, but see the LICENSE file for restrictions.