Do you like short works of fiction? Do you like things that are effin' strange? This may be the website for you.
Every now and then, you find a site that starts out as an entertaining diversion and quickly comes to vacuum up more and more of your free time. I've found the SCP Foundation to be just such a site. It's a short-story wiki, a database of hundreds upon hundreds of bite-sized descriptions of monsters, immortals, objects of power, inexplicable phenomena, and various and sundry "defiances of natural law."
There's the coffee vending machine that dispenses a cup of whatever you ask for, even if it has to reach out to other planets to do so. There's the woman with a full Nazi German war bunker somehow contained within her nose. There's the red stone that lets you walk through mirrors into an abandoned world, as described in a genuinely unsettling series of accounts that I do not recommend reading late at night. There's the zombie virus from outer space.
The central conceit of the SCP Foundation is that it's your basic shadowy multinational organization dedicated to the study and containment of all things paranormal. To that end, the stories take the form of case studies, incident reports, and lists of precautions, written in a clinical, detached tone that's impressively consistent. (In reality, the "Foundation" seems to have begun life as a series of posts on 4chan, though by now it's got a culture and register all its own.)
It would be easy for a site like this to descend quickly into grim, top-this humorlessness, but fortunately that's not the case. A lot of the stories have a healthy sense of whimsy (see SCP-131, the affectionate, catlike "Eye Pods"), and the Foundation researchers are usually depicted as a prankish, haphazardly principled lot, in what might be a sly comment on the real-life bureaucrats who make the decisions that shape our world.
A good place to start is the Secure Facilities page, where you'll find brief descriptions of the Sites and Task Forces at the Foundation's disposal. These are expertly written to rev the imagination. (Sample entry: "Mobile Task Force Rho-5, aka 'Stitch In Time': Assigned to SCP-276 for the purpose of repairing temporal anomalies. Currently somewhere in the Victorian Period.") After that, you are encouraged to dip into the full index, picking up the site vocabulary ("Euclid," "Keter," "O5-X," "[REDACTED]") as you go.
It's worth noting that not every story is good; some are derivative, or dull, or just not successful at what they're trying to do. But the fun of a site like the SCP Foundation is that there's something for everyone. I have a couple of favorites—SCP-033, the "Missing Number," is close to my heart, and SCP-231 plays with elision and suggestion to produce an effect of indelible creepiness. But you'll probably find your own that you love, and I suggest you note it in the comments if you do. There's a lot of good work here, and it deserves to be recognized.
Top image: Untitled by Izumi Kato, from the collection of Ryutaro Takahashi, via Little Boy.