Vines

Vines (sometimes called Ivy) were included in the Adventure Update and are placeable on walls and ceilings. They also hang on leaves in Swamps. As of Beta 1.8.1, they take as long to break as leaves and drop nothing. However, they are collectable by using shears. Vines can now be climbed as in Minecraft 1.2.2, Players must first push into a block then look up, similar to climbing ladders.

Trivia

 * It greatly reduces the chances of getting fall damage in the Jungle, as it absorbs all fall damage.
 * When descending vines, if the vines overhang an empty space, it is possible to continue descending from the vines as if there were a block there.
 * When harvesting hanging vines it is important to remember to work from the bottom upwards for maximum yield. Breaking a hanging vine will destroy all sections lower than it but will only yield one piece of vine, regardless of how many pieces were destroyed.
 * If blocks are placed near a vine (such as the case of having a house built in a swamp biome, for instance), vines will start growing on the wall.
 * Vines can grow in any direction when placed on a block. When there is no block next to the block where the vine is on it can grow to the side of the block. The vine can even grow up.
 * Like signs and ladders, only one vine may occupy a given block. Now, more than one vine can occupy one block. Talk
 * When vines grow down they don't need a block below them to attach to, however, the topmost vine needs a block to attach to.
 * Vines cannot be placed directly on blocks like chests, Crafting Tables and cobweb. However, they can be grown around them.
 * Vines are flammable.
 * Vines are most effectively destroyed by water. Vines on a mountain, for example, can be easily flushed away by putting a water source block on the mountain.
 * Vines block mobs' line of sight, so it is possible to sneak up on mobs by hiding behind vines. Because of this, Endermen can be safely viewed through vines.
 * You cannot place vines directly below blocks but they can grow under blocks.
 * When placed on a wall that is touching the bottom of a block, the vine will also attach to the bottom of the block.
 * Vines have the same color as foliage has in a specific biome.
 * Vines have the ability to support sand and gravel. However, unlike other such blocks, vines will be destroyed when sand or gravel attempts to occupy the space adjacent to the vines.
 * In the 1.8 Pre-release if the player attempted to place Vines in the Nether the game would crash.
 * Sometimes when players place vines on cobblestone, it might be mistaken for mossy cobblestone.
 * At PAX, vines hadn't been given a name, and a mouse over would not give a title. This implies that they didn't have an official name at the time.
 * The player is able to walk through vines.
 * When held and viewed through third person view or held by another player, vines have a gray texture, like lily pads.
 * Multiple vines can be in the same block space, but count as one block for most purposes. Talk
 * The player cannot place vines under another vine where there is no blocks (Just how vines hang off the trees that are naturally generated.) To do so, the player will need to put a block under the block of the original vine, place the vines on that block and destroy it, the vines will then hang off the first vine. If the vine that is on the block is destroyed, each other hanging vine will be destroyed as well.
 * Vines can spread onto portal blocks.
 * When you set vines on fire, sometimes the block disappears and smoke appears, then other blocks around it catch fire.
 * Vines make the same noise as grass when stepped on.
 * Vines can be put in front of doors to make secret doors.
 * Vines can be found in two colors, the common darker variety, and the rarer brighter variety found only on trees that hang over water, the appearance is the only difference known of.
 * Vines seem to have a bug where they grow behind paintings.
 * Vines will disappear if a torch is placed on them.
 * If you keep placing vines on the same square, it will use up your vines but only place a single one.

Griefing
In the 1.8 pre-release, the rate at which vines spread made them the newest tool for griefers.

On Sept 13, Notch said that he would change the vines to spread infinitely down, but would not grow other directions if there are more than four other vine blocks nearby.