Translation: I go on these all the time and actually I think you should too.
In descending order of danger:
If there's one thing teachers agree on, it's "don't go on Wiki for homework". This is actually the only time you should actually use Wiki, because you have a deadline breathing down your neck. Otherwise, you might find yourself deep in the workings of Vietnamese politics, with twenty tabs open.
Fun fact: As an experiment, I once started out at popcorn and managed to get to the Chinese Warlord Era within 3 minutes.
4. Archive of our own
Despite their occasional weirdness, those tags do come in handy when seeking out a rare-ish character/shipping. Unfortunately, the ease of finding stories in your niche means that you will leaf through 18 pages of results, then go to another tag and leaf through another 20, and so on.
It also really, really sucks when you find something with the perfect description and you click into it ... only to find it it has no paragraph breaks. *growl*
3. Conjugal Felicity
Describes its aim as "the sporking (detailed, page-by-page, snarky critique) of poorly written books". There are lots of genres to choose from, including MG, YA, erotica, and weird stuff I can't even categorize, but all of them are so hilarious that I got hoarse from laughing the first time I discovered this site. Personal favorites are the Maradonia Saga and Ruin Mist chronicles.
It's so addictive that I took a break from writing this just to re-read a couple of articles.
Does this actually need explanation?
It's doubly worse if, like me, your time zone is 12 hours away from most of your Internet friends. Under normal waking hours, everyone wakes up when I go to sleep, and everyone sleeps when I wake up. Because Tumblr, I have accordingly adjusted my schedule to be awake at midnight AND 7 AM even though it's summer break. *facepalm*
1. TV Tropes
Describes itself as "catalog of the tricks of the trade for writing fiction". It will ruin your soul, your life, and your liver. It even admits it. Once you click into this site, you will never again properly enjoy a book, TV show or movie without thinking of tropes. Worse, you will have a hundred tabs open. The number of tabs exponentially increases with time, and the exponent is higher every successive visit.
The final straw is when you start using "lampshade" or "Princess Classic" in your literature essays. Hmm, sounds like a Student's Guide post to me ...