Ghast

Ghasts are very large jellyfish-like mobs that float through the air and shoot fireballs from their mouths. Their bodies are 4 by 4 by 4 block cubes with 9 tentacle-like limbs hanging from the bottom. They first appeared in the Halloween Update. Ghasts are able to spawn anywhere in the Nether, as long as there is sufficient space and the game is not in Peaceful mode.

Behavior
Ghasts normally float around with their eyes and mouth closed. They open their eyes and mouth when attacking. They make a chirp-like "heeeYAAAAII" noise when shooting and scream loudly when taking damage. The sounds that Ghasts make have a minimum intensity regardless of their distance, so they often sound closer when they are actually far away.

If a Ghast's fireball hits a portal (or close to it) the portal will be closed, requiring re-ignition. If the player manages to get a Ghast to fire at the portal while it is closed, it can be reignited using the fireball. Blocks originally ignited once struck, adding an extra hazard to fighting Ghasts; however, the fire is extinguished immediately afterwards because of a bug.

The minimum block resistance required to absorb all blast force of the Ghast's fireballs is 20.17, meaning that fences and other less resistant blocks will be destroyed in a fireball explosion.

Ghasts will not fire at the player without line-of-sight, and this is blocked by glass, leaves, portals, or webs making safe scenic paths through the Nether a viable possibility. When not moving, Ghasts within firing range will face the player, giving the impression that they can see through transparent materials. Ghasts can draw line-of-sight through curtains of lava, and their fireballs can pass through it meaning players near a pillar of lava can be struck without seeing the source.

Ghasts drop Gunpowder when killed, but this can be hard to collect - as it is the only mob that flies, the gunpowder may fall into lava or an inaccessible area.

Fighting Ghasts
Ghasts attack with projectile fireballs, which don't aim exactly at the player, but slightly above their head. The player can reflect these fireballs by hitting them with anything before they impact, even bare hands, arrows or fishing rods. It is not possible to kill Ghasts by reflecting their attacks back at them even considering their low health and high attack damage, even though you can still hear the Ghast's "hurt" clip being played when hit. You can make a fireball easilly reflect in the direction you want by shooting an arrow, fishing rod, or snowball at the fireball. The Ghast hitbox includes the tentacles, so aiming for the area where the tentacles meet the body increases the chance of a hit.

Ghasts may be hurt with normal weapons if the player can get close enough or has a bow and arrows. It is possible to use a fishing rod to draw Ghasts in near enough to hit with a sword. A close Ghast is also easier to hit with a bow, though the drawback of this method is less time to dodge fireballs. When aiming your bow be sure to lead your shots as Ghasts can quickly change trajectory and move quite fast. Depending on how far away the Ghast is it may be necessary to aim above the Ghast as well to gain extra range.

Ghasts are often considered the hardest mob to kill, especially as they can spit their fireballs from long distances (approximately 30 - 40 blocks away). Due to them being so common this makes the Nether a hard place to survive in. Ghasts can also break blocks from under the player, often dropping them into lava or causing death through falling damage.

Ghasts may also be set on fire, but fire and lava don't actually damage them. A similar effect is seen with zombie pigmen. When a Ghast is caught in lava, its movement is impeded, and then they can be easily targeted by arrows.

Much like using neutral or enemy mobs as shields against arrows fired by skeletons in the regular world, it is feasible to use zombie pigmen as shields against Ghasts in the Nether. It is possible to injure or kill zombie pigmen in this fashion without ever making them hostile to the player.

It is disputed that Ghasts can be killed by lava or fire if continually re-exposed. This is achieved by a Ghast floating in and out of lava. There have been reports of zombie pigmen dying while in lava, due to the fact they "bob" in and out of lava, which raises doubt that Ghasts and zombie pigmen are completely lava/fireproof.

Despite being fireproof, Ghasts and Zombie Pigmen are capable of drowning if under water or lava for too long. This means they, like the player and other mobs, require air to live.

Mining
The Ghast's fireball can destroy all natural blocks in the underworld. So you can use Ghasts to help you mine. In order to do this, you have to know how to make a fireball at the spot you want, since Ghasts don't aim exactly at you. When you do this, the Ghast destroys the blocks, and you can safely collect them. The dangers to this method include the lava, ledges, etc. and fire when the bug mentioned above is fixed.

Drops
Beta:
 * 0-2 units of Gunpowder

Trivia

 * Ghasts can "cancel" their fireballs if the targeted player has an obstruction between the Ghast and themselves.
 * Ghasts aim at the camera, not the player. Going into 3rd person mode and having a Ghast fire a fireball at you will go over the player, given the right camera angle. This also means that they will continue to fire at the player if they are dead, but is still in the respawn/quit window.
 * Ghasts can cause other monsters to attack it but it will only attempt to attack the player.
 * If you manage to hit a ghast with its own fireball, it is dealt very little damage compared to the damage done when it hits a surface, another mob, or you.
 * If you successfully reflect a Ghast's fireball, and if you "aim" right, you can get the fireball to strike and kill a Zombie Pigman, without even making them hostile.
 * Ghasts are not affected by light, and thus appear the same brightness anywhere.
 * When a Ghast shoots a fireball, it looks like a flaming snowball, though its effect is different from that of a normal snowball.
 * If the cursor is over the Ghast's fireball the box around the equipped item will light up.
 * Notch added a small chance for Ghasts to spawn in the Overworld near Portals in Beta 1.5 . A Ghast spawned outside of the Nether will fly everywhere, mainly above the clouds. Can even spawn extremely rarely without a portal.
 * Ghasts are the biggest mob, being eight times as big as the biggest variation of slime.
 * A fireball will not disappear if in flight when switching to peaceful.
 * If a Ghast is trapped in a cube, it will shoot fireballs through the cube. This is because like all other things that shoot, it spawns the projectile one block in front of it.
 * On SMP, Ghasts do not make the firing noise when attacking.
 * Ghasts' explosions are less "connected" to them in SMP than in SSP. For this reason in SMP mobs damaged by a Ghast's fireballs will not attempt to attack the Ghast and Ghasts that are either spawned or follow a player into the surface world (see below) can become divorced from their explosions and become an essentially peaceful mob. Similarly, Ghast explosions can sometimes follow a player into the real world, without any nearby ghasts, but these normally only appear in the form of apparently random explosion noises not connected to actual damage causing explosions.
 * When a player in an SMP server dies near Ghasts or is teleported out of the Nether to the surface world (not via a portal) there is a chance the Ghasts will follow them to their new location in the surface world. These Ghasts may act like normal Ghasts but could also behave radically differently than normal. Changes in teleported Ghasts include the Ghast-less explosions and Ghasts without explosions mentioned above. The teleported Ghast may also freeze in one location or "stalk" a certain player (moving closer to a player until they touch like a normal hostile mob) but both of these are extremely rare.
 * Ghasts are the only mob in Minecraft that do not change damage based on difficulty.