Spider's Silk

Spider's Silk is dropped by low-level spiders, from level 16 to 36. It is used as a component in recipes from several professions. Its rarity combined with the value of the resulting crafted items make it a valuable drop easily sold on the auction house.

Source
Spider's Silk can be farmed from many species of spiders. The best drop rates come from the spiders of the Arathi Highlands. The Giant Plains Creeper drops Spider's Silk at approximately a 10% rate.

For level 20-30 players, a good place to farm is Thelgen Rock in south-central Wetlands.

As an ingredient
Spider's Silk is used in the following recipes:

Leatherworking

 * Toughened Leather Gloves (level 135)

Tailoring

 * Lesser Wizard's Robe (level 135)
 * Robes of Arcana (level 150) (used in a Warlock quest to create the )
 * Spider Silk Slippers (level 140)
 * Spidersilk Boots (level 125)
 * Spidersilk Drape (level 125)