Scribd

I chanced upon Scribd a couple of week ago, and I can't recommend it highly enough. It's basically like a giant library, but its got loads of fan created 40k and Warhammer stuff. You'll probably also find Codexes on there which is useful for a quick check on an army you might be facing. I wouldn't have said it would replace buying a proper codex or rule book (GW still need the money to keep producing stuff after all), but its useful to check out something before you buy it.