Spider

The spider was the second mob added to the game. The spider's eyes glow red and can be seen in the dark, making their appearance rather creepy. Unlike most other hostile mobs (1x1x2 box), the spider is a flat mob (2x2x1 box). It cannot enter the same narrow spaces that zombies and other tall mobs can, but it can crawl into places where the ceiling is too low for any other mobs. Seeing as they are much larger than most hostile mobs (1x1x2 vs. 2x2x1), they can be hit rather easily with arrows. A lone spider is easy enough to deal with, but a pack can quickly close distance on the player due to their jumping and wall climbing abilities.

As of the Beta 1.2 update, spiders can climb vertical walls (this includes fences). One way to deal with this is to make sure the roof is accessible through only a 1x1 hole (or a 1x2 high doorway... door is optional) that spiders cannot drop through. Look here for more tips.

Another way of stopping spiders climbing walls is to make a ring of ladders part way up the wall (about 4 blocks up to be out of reach), because the edges of ladders block movement.

Spiders are not always aggressive. When in zones with lots of light spiders become neutral and will not attack unless they are attacked. When spiders are in a neutral condition, their eyes do not glow red and the player will not receive damage if they touch the Spider. Spiders become aggressive again when night falls or if it moves into darkness. If they were chasing the player during the night, they will continue to chase them during the day.

Spiders in heavily forested areas may have enough darkness to turn hostile during the day. Spiders in a room constructed entirely of glowstone will be neutral no matter what time of day it is. Spiders can also turn neutral on torches but will return to being hostile the minute they move off the torch.

Spiders make a sound like someone slurping a straw or a mix of a screech and paper rustling and their distinctive noises can warn the player of their presence.

Spiders can spawn mounted by skeletons. These spider jockeys can also climb up vertical walls.

Hunting
Spiders can prove difficult to hunt, but because they drop string, they can be a very rewarding kill. Proper equipment for hunting spiders includes a weapon, like a sword or bow, and some type of armor. However, due to their great ability to close distances, bows are preferred by many players as spiders present a large target. When attacking a group of spiders it is recommended to pick off the ones that aren't too close to others in order to avoid startling them all at once. Spiders are the fastest mobs in Minecraft, and should never be underestimated.

Dungeons that spawn spiders can effectively be rigged with cactus blocks to harvest spider string. Because spiders are two blocks wide, they cannot pass through the gap between two cactus blocks (or a cactus and a wall) and will press themselves to death against a cactus while trying to get to the player.

History
Spiders were first introduced to the game in Survival Test and didn't change much until the January 13th, 2011 update. In Survival Test, they moved at the same speed as in Beta, but they could not jump as far. Killing this mob netted you 105 points.

In the January 13th, 2011 update, Spiders were given the ability to climb vertical walls made of any block, and they were also given the ability to see through walls to make their wall climbing ability more potent. Creepers used to have the ability to see through walls before it was given to the spiders.

Drops
Survival:
 * 105 points

Alpha and Beta:
 * 0-2 pieces of string

Trivia

 * In the terrain.png image, there is a spiderweb to the left of the flowers. It is unknown what this is or was to be used for, but it was probably spider-related.
 * Spiders can jump 2-3 blocks.
 * Causing a spider to take fall damage or damage from a cactus after attacking it during the day causes it to become neutral again.
 * Spiders don't charge when climbing a vertical surface, making it easy to ditch while it's chasing you over mountains.
 * Spiders were once brown.