TreeClimbing makes all logs, when part of a tree, act like ladders, and leaves - again, when part of a tree - nonsolid when collided from below.
The combination of these two things means you can now climb up trees' trunks, through their leaves, and stand on the canopy. This is enormously useful when paired with something like the OpenBlocks hang glider.
There is significant effort put into mod compatibility, so it should work for most if not all modded tree types.
You can also sneak while on leaves to fall through.
