The SCP Foundation Warning: Memetic Hazard. You will not escape. You will not survive. Classification: Safe, but strict containment protocols are in place. Seriously, though. You want Artifacts? Here’s the place to go.
Project Gutenberg Public Domain books. I recommend the colored “Fairy Books” and Dunsany.
Internet Archive More books. Annoying renaming schema, but an amazing depth and quality of content.
The Medieval Bestiary What it says on the tin. Go here when you just want something to make your players go, “WTF”.