Lexicanum

The Lexicanum is an unofficial wiki based on Games Workshop's Warhammer Fantasy Battle and Warhammer 40,000 product lines. They're fully dedicated to record every bit of fluff there is, EVERY BIT. They do however refuse to record any kind of crunch on Warhammer characters for some reason, which is both bald and foolish as more Warhammer players prefer to look up the crunch on units rather than fluff.

Regardless, the Lexicanum is still a good place to look up fluff on anything Warhammer as they've recorded almost all of it, just don't get confused in basing your army on lore than stats when playing table-top wise.