Browser Narcotic

Any web site that results in you opening a dozen tabs in a single session and using up hours of your time. Like this one. Ending up on a Wiki Walk is common at such sites, but not universal. The trope name comes courtesy of Xkcd's alt-text on this comic.

Compare Archive Binge, which is linear in nature and does not require opening dozens of tabs.

These sites are notable offenders:

One of the many ways All The Tropes will ruin your life.
- This may also happen at TV Tropes.
Wikipedia
Reddit
Cracked.com
Dark Roasted Blend
Deviant ART
Digg
Everything2
Facebook
Tumblr
Fanfiction.net The good ones at least... and badfic too, arguably.
Fark
Oobject
Damn Interesting
Forums can end up as these if there are enough interesting threads.
The Polish site Wyhacz.pl is was a news service devoted mainly to various instances of citizens being screwed over by bureaucratic incompetence or corporate dishonesty. It's surprisingly fascinating.
The Let's Play Archive: Oh, you've just discovered the Let's Play phenomenon and spent several hours following an LP of your favorite game? No worries, we can recommend LP's from the same genre / author that are sure to interest you. And once you're done with those, we have more recommendations...
The Cheezburger Network
The SCP Foundation can do this, as some of the most popular articles include experiment logs involving other SCPs. The site is trying to minimize this, however. Just watch your step, because you're walking through a minefield of Nightmare Fuel. Really high octane stuff.
Snopes
StumbleUpon.
Twitter
Orion's Arm, hoooo boooy.
Yahoo!.com (the main page that features news articles).
YouTube
AniDB
Hardcore Gaming 101. Hundreds of in-depth articles on a staggering variety of niche, overlooked, or import-only games? Updated semi-regularly? Not only are the articles a fascinating way to waste a few ~~minutes~~ ~~hours~~ weekends, a whole new level of procrastination can be reached if one decides to make trying the games featured on the site a regular habit.
Pixiv, thanks to its recommendation feature being a little too good, tends to induce Wiki Walks. Heaven help you if you start to browse for fanart of one of the more popular series, like Touhou, Hetalia, Pokémon, Vocaloid, or Inazuma Eleven, each of which will get you over 150,000 hits.
Everything Is Terrible