Wahapedia
Wahapedia.ru is a rules repository for Warhammer 40k, Age of Sigmar, and Kill Team, notable for presenting you with all the rules needed for play, from the Core Rules to Codices to FAQs and White Dwarf expansions. If you're wondering if any of this is legal, it is absolutely not, but manages to avoid attention because of its relatively obscure niche and the fact that its a Russian site operating on Russian servers, frustrating the effort of the capitalist dogs British to get it taken down.
In spite of its lack of legality, Wahapedia still manages to be easier to read and understand than GW's own official App, being very easy to read and with convenient scrollovers for double-checking rules and Keywords; extremely convenient for those getting into the game, those trying out new armies, and the poor playing poorhammer. IRL, your mileage may vary on how much your playgroup will accept you for pulling it up on your device as a rules reference: obviously forbidden in LGS, GW locations, tournaments, and in the basements of those that have gone off the deep end and invested their entire life savings into plastic and resin, but indispensable for first-timers and other Virtual Tabletop Gamers.
On that note, Wahapedia is invaluable for those living in Russia and other regions with a low GW-presence but still have a healthy number of online shut-ins elegan/tg/entlemen and ca/tg/irls; not all of those recasts are made for the West, after all, and GW products are even more niche and overpriced in Asia, where they need to be acquired through the same arcane pattern of mark-ups and specialist retailers that a Weeb would need to buy their plastic waifu models outside of Japan.
Because this is all the work of a single Russian, it will obviously take time to update, but if/when it does, a certain other wiki inevitably gets its own updates.