Rubber Tree (RedPower2)
RedPower2 Rubber Trees are incredibly large, easily containing over 256 blocks of Rubber Wood a piece. They generally only grow in Jungle biomes.
The only current purpose of Rubber Trees is for their (Jungle) Rubber Wood which differs from "normal" Rubber Wood in that it can only be crafted into sticks (1:8 ratio) or each piece can fed through an Extractor for a 1:1 exchange for Rubber.
How to grow
In order to grow one of these massive trees, you must make enough room for it to grow. These trees need at least a radius of 20 blocks to grow. Now, place a single sapling in the room you just made.
You can not use Bone Meal on these trees. They take an unusually long time to grow, ranging from one hour to six (Real time). The easiest way to ensure one grows is to bury yourself in dirt near the chunk with the Jungle Rubber Tree sapling and leave it loaded overnight.