Generators: This site is the bomb. Generate NPCs, random encounters, loot piles, traps, encounters, full dungeons even, magic shop inventories, weather, and so on.
Monster Finder: Handy little tool for GMs if you want to scour the field for monsters based on determined parameters.
NPC traits: 1,000 NPC traits. Site also has a one sentence NPC generator that is actually not bad.
Online dice roller: includes a 4d6 drop lowest reroll 1s set roller.
Second Edition Archive — Massive .pdf that contains nearly 6k pages of published 2E material.