Bukkit

Giant Trees will naturally generate trees in some of the biomes. To create a tree on demand, use the /gt command. All the trees you can spawn are in the plugin's data directory. The files are named tree.TYPE.xml and tree.TYPE.root.xml. The root files will create the tree's root structure. To grow a giant tree in creative mode, surround an emerald block with two rings of saplings and fertilize them with 64 bone meal.

To make new tree species, double click on the plugin's .jar file or use the tree-edit command from the server console. Save your work in the plugin's data directory.

Works on my machine.