http://www.cracked.com/: I sometimes look up to find that I've spent an hour or more reading article after article on Cracked. A great thing about Cracked is they cite their sources, so you can in fact try to see how deep the rabbit hold goes.
damninteresting.com : I really wish they would update more. The stories and events that they relate on their site are fantastic fodder for stories and, well, musings. Like Cracked, they've been a leaping off point for me when I've thought "Well I've never heard of that" and went to dig.
Wikipedia.org: It's important to not use Wikipedia as your only source. However, if you need to see a picture of some random thing, or want a good summary with links to reputable sources, it's a good place to start. Plus, reading Reference Desk questions are amusing.
Pantheon.org :Otherwise labeled as "Encyclopedia Mythica", a great source for mythology around the world, so you can make classical references to things and look super-smart. Or, you know, look up what actually happened to Eurydice, when you think a book you're reading got it wrong.
http://www.nationalgeographic.com/ : A lot of full articles and videos are on this site. Additionally, you can buy all of National Geographic ever on disc, which updates on your computer with new articles every month. So you can have it all! The pictures are iconic, and the articles are in depth.
WeirdNJ.com : I'm from New Jersey, and I think reading Weird NJ magazine (well, and having a sleepover with girl scouts) got me really interested in urban legends and urban exploration. While I personally don't want to go to abandoned sanatoriums and photograph them by moonlight, some people do! There used to be far more on the web site, but I guess as years of the magazine have come out, they've phased some of that away.
Snopes.com : Full of urban and Internet legends, and a great place to go when you get a sketchy email and go ".....really?" There are also forums.
TV Tropes: Practically any literary device or convention that writers use. Though the name is "tv" tropes, it covers books, movies, video games, and practically every media you can think of. Snarky and irreverent, and some great wisdom.
The SCP Foundation: A friend just told me about this the other day, and it is fantastic. SCP stands for "Standard Containment Protocol", and each numbered..."thing"?...has its own story. A great urban legends/faux black ops style of writing and recording, and rather inspirational. It might be considered "Meta"? Yah, maybe.
Roleplay Portal: This web site is useful to me because it has all of the Classic World of Darkness and much of the New World of Darkness right there on the web. So, if you're a tabletop gamer, and into the World of Darkness (old or new) you can have merits, flaws, rules, etc. right at your fingertips rather than flipping through ten or more books.
Pathfinder SRD: Similar to the Roleplay Portal (and perhaps updated more frequently), this website collects all the rules from the Pathfinder role playing system, which leapt off from (and is compatible with, supposedly) 3.5 Dungeons and Dragons. Again, nice to be able to see complete lists of spells, feats, and classes without flipping though a bajillion books.
Drinking Cup: I've only begun to peruse the articles here, but the site's tagline is "A brief history of the world through the drinking cup". So, I've read about Coca Cola (and Pepsi), coffee shops, and absinthe so far.
NASA: It's amazing how much is on the NASA web site. Pictures, articles, ebooks, there's even NASA TV, which will frequently live stream launches.
NASA on Soundcloud: want to hear lightning on Jupiter? Want to hear an Atlas V launch? It's there. This is one of those website discoveries I was very excited about, but nobody else I knew was.
io9: I always find great historic and frequently science-y stuff on io9 that's right in my wheelhouse. I don't like the page format very much, though, and wish they'd use something else. Still, it's very much worth it for the content.
Bible Gateway: They've got something like 100 different Bible translations you can peruse and search. Want to know how many times Cain was mentioned in the Bible? Doable. You can also look at verses side by side in different translations, which can be very interesting indeed. Some interviews and study stuff as well, but I haven't delved into that and cannot speak for it's quality in any way.