Lexicanum
The Lexicanum is an unofficial wiki based on Games Workshop's Warhammer Fantasy Battle and Warhammer 40,000 product lines, like this wiki; it uses the wikimedia format, distinguishing it from the Warhammer 40k Wiki wich uses Wikia. 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 most Warhammer players prefer to look up the crunch on units rather than fluff, though there are a large amount of people who don't play the game for various reasons (cost being one of the biggest ones if memory serves, Warhammer 40k is very fucking expensive) but are interested in the fluff. They also have very a stringent policy on everything from articles and images to personal comments on talk pages, so much that you'll rarely see any non-Warhammer related humor in any of their talk pages or articles.
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.
When compared to the Warhammer 40k Wiki, the Lexicanum's articles tend to be shorter due to Inquisitor S's paranoia, but are more numerous, so while the WH40k Wiki's Carnifex article is much longer and more detailed than the Lexicanum's, the Lexicanum has Articles on Hive Tyrants and Heirophants, which the WH40k wiki does not. If you want every bit of information that has been collected about a specific WH40k, the Warhammer 40k Wiki is the better bet; though there is the caveat that the WH40k wiki an article for that particular thing. But if you want to be sure of finding an article for even the most obscure wh40k topic, the Lexicanum is your wiki. One notable difference between the Lexicanum and WH40k wiki is that the Lexicanum has been just barely updated to have Dawn of War 2 Retribution information, while the WH40k wiki has yet to have a single edit that displays any information from Retribution.
The WH40k wiki and the Lexicanum of course, have something of a rivalry, but there is little doubt that the Lexicanum is the better known of the two sources.
Inquisitor S.
The Grand High Marshall of Faggotry himself, Inquisitor S. Is the head admin on the english Lexicanum. He is against all forms of progression when it comes to the updating of the sites information and pages, and regularly bannes and blocks users who make contributions other than fixing grammar and punctuation, he makes it his job to ensure that zero new information is added to the site as to keep the site in the stagnent, un-altered, and "perfect" state is in now. He is quite likely trying to emulate Games Workshop's own stagnation. While his level of paranoia has ensured that ZERO fan-fiction enters the site, it is also the reason that the site is currently in a theoretical dark age where most of the information on the site is over 3-4 years old and stale and the site itself is severly out of date. This is most evident in the fact that the English Lexicanum (the one that he runs) contains only 57,000 articles wheres the German Lexicanum (the far superior version of lexicanum where Inquisitor S. got perma-banned from for being a faggot) has over 10,500, and is also why the Lexicanum's rival; the WH40k Wiki has much longer and more detailed pages on average than the lexicanum.
NOTE: While the 40k wiki's pages are of far superior quality it lacks the number of articles that Lexicanum has and has a problem with allowing fan-art on the site (a big no no for any serious information wiki)
tl;dr: Make an edit that contributes the the site and you'll either get your edit reverted or end up banned. In fact, heres the link to his talk page...
Troll away