I made a comment to a friend the other day about time sinks, so I thought I'd document a few of my favorite here, in case anybody is interested. Please comment if you have your own recommendations!
The SCP Foundation. Secure, Contain, Protect. The world is not what it seems, and the SCP Foundation is there to help keep things sane. Well, sane-er. Sane-ish. This isn't a web comic, but a shared mythology site where many authors contribute to help describe the SCP Foundations and other world-spanning organizations dealing with the paranormal, the occult, the extraterrestrial, and the Just Plain Weird.
TV Tropes. The all-devouring pop-culture Wiki. Enough said.
/r/nosleep. Just found this a few weeks ago. A Reddit forum for posting original horror stories. Read "Search and Rescue" and "Room 733", definitely. The quality of the stories can be all over the map, but there's some good ones in there.
Narbonic. If you haven't burned several hours reading this yet, go ahead and start from the beginning. The adventures of a middle-aged mad scientist and her favorite henchmen/experimental subjects. The artwork was never bad; a little rough in the earlier strips, but it improves dramatically as time goes by. The storyline starts off nicely, mutates into awesome and then kicks it up a notch or eleven into uncharted territories of incredible.
Skin Horse. Essentially a follow on to Narbonic, with a few crossover characters. If you find the idea of a secret government agency dedicated to providing social services to the creations of mad scientists intriguing, this is the webcomic you've been waiting for.
Schlock Mercenary. A very, very long, very large scale SF webcomic. The artwork starts off very rough, but improves dramatically over the life of the strip, and the story lines and characters are amazing. Author John Ringo has done a series of books ("Live Free or Die", "Troy Rising", "The Hot Gate") set in the early days of Humanity in the Schlock-verse.
Digger. Dead gods and wombats. Won the Hugo Award for Best Graphic Story back in 2012. It's a truly delightful story, and one that I've re-read more than a couple of times, finding something new in each pass.
Order of the Stick. OotS started off as a Dungeons & Dragons funny comic, and... as they say, Things Happened. Rich Burlew is a fantastic artist with a very distinctive style, a great storyteller, and man... the *story* that grows out of those first few gag strips? Amazing.
Sluggy Freelance. One of the Ancient Elders of the webcomic world, the strip alternates between silly, serious, trivial and terrifying. Plus, it has a murderous, sociopathic, talking lop-eared rabbit. What more could you ask for?
Freefall. A SF webcomic. With DOGGY! Oh, and a morally-challenged space quid, about a bazillion robots, a veterinarian, and a story arc that is really, really quite impressive.
A Redtail's Dream. A incredible, very visually impressive story based on Finnish folklore by Minna Sundberg.
Stand Still, Stay Silent. Minna Sundberg's new story, about a world-ending virus and what comes after: zombies, trolls, giants and things that go bump in the night... sometimes, very, very forcefully.
Basic Instructions. You're living your life - do you want to know how to live it *well*? They you probably don't want to follow Scott Meyer's advice, unless you want your life to be more... interesting than it is now. Eh. You know what? Forget that. Take everything he says as a lesson in life. You might have some problems, but the rest of us will be amused.
Broodhollow. A sweet yet creepy webcomic by Kris Straub. The artwork is wonderful. Still in progress, and just returning from a hiatus.
Experience Boost. Not a WoW comic. Nope, nope. This is all about... World of Quests! Yeah! Totally different! Except in all the ways that make you laugh.
Girl Genius. by Kaja and Phil Foglio. Yes, FOGLIO. Go. Read. Mad science means never having to say your sorry (though it probably is polite, dear.)
Goblins. What happens when a group of goblins, guarding a poorly secured treasure chest, decide that THEY can be adventurers, too! Starts off kind of rough around the edges, and without much of a plot, but really grows from there, and ends up going into some dark places (and planting lanterns there, in a very satisfying way). Updates are not all that regular, but the wait is worth it.
Full Frontal Nerdity. A comic about gaming and gamers that really is really about... well, gaming and gamers. But, like, done really, really well. By Aaron Williams. who also does Nodwick, which is just as good, if not better.
Nodwick. Every party of loot-crazy adventurers needs a henchman, right? Well, that guy's Nodwick.
Use Sword on Monster. From the creator of Nodwick and Full Frontal Nerdity. What happens when universes collide? Well... heroes. Just not the heroes that you'd necessarily want.
The Whiteboard. Alaskan paintball, things in the fridge, romance, particle accelerators, and generalize silliness. Definitely worth a mention as Doc, the main character, has appeared as a cameo in more than a couple of other comics on this list... so if other web cartoonists think it's cool enough to read, you should read it, too. Obviously.
JL8. Ever wonder what the Justice League would have looked like in middle school? Wonder no more!
The Mansion of E. The artwork isn't as much bad as it is minimalistic; but the storyline and the world are... wow. Creative just isn't a big enough word to describe it.
Wondermark. An Illustrated Jocularity.
XKCD. You're probably already reading this, even if you're not. At least, that's what multiverse theory suggests. You should start reading it so you end up in one of the good universes.
Xylobone Tomes. A lich, a plant girl, and... well, strangeness is afoot. Lovely black & white artwork, and what seems to be an intriguing story so far.