TV Tropes is a wiki known on CE for being very addicting. The basic concept is collecting tropes, which are conventions and devices used in creative works. It differs from other wikis in that it doesn't require its users to be formal (except that the admins and mods sure are afraid of appearing to be tolerant of pedophiles; they even deleted the history log for the former trope), and specific works just have to exist to qualify for their own page, as opposed to being well-known or famous. It can also cover other topics like online personas or horrible logic fans of a franchise can use. Common topics about TV Tropes on CE will complain about hundreds of tabs open on their browser, and it's very easy to spend hours on end browsing the site.
TV Tropes is usually the first website on CE when a topic is made asking for addictive websites, leading the TC no choice but to inb4 TV Tropes.
If you still don't understand why it's so addicting (it's kind of hard to put in words), wait until you have about 6 hours to kill and try reading the page for your favorite game, show, movie, etc.