How do I make big jungle trees?
Planting a single jungle sapling will create on of the small jungle trees.
Placing a group of 4 jungle saplings in a square (and using bonemeal) will make a large jungle tree. I'm not sure if the bone meal is required, or just speeds it up.
Apparently not all the saplings need to be jungle saplings, just the one you use bone meal on.
This was patched in MineCraft 1.2.4, and no longer works; all 4 saplings need to be jungle saplings.
For completeness, there is another way to create an arbitrarily tall skinny tree:
From the ground, create one block tower of wood from the tree that you are trying to grow (for example, birch wood for a birch sapling).
When you've reached the approximate desired height of your tree, place a block of dirt atop your tower.
Place the sapling in the dirt.
Apply bonemeal to the sapling.
Replace the dirt block with a wood block.
Ta da! You've created the equivalent of a truffula tree!
Yes, big trees are random but any sapling can be forced to (eventually) create a big tree:
If a sapling is surrounded by a 2 block tall tube it guarantees that it will, eventually, grow into a big tree. The tube forces the small tree growth to always fail each attempt. However, this takes far longer to produce a mature tree, due to the low chance that a sapling will try to grow into a big tree.
From Wiki.
Although remember that trees are just generated structures. You can harvest leaves with shears and make your own giant trees!