And, of course, briefly explain why it rocks.
My entry will be the Internet Sacred Texts Archive. The first entry in its FAQ explains its value quite well:
I came across this site while doing some research for the novel I am, at this very instant, avoiding working on. My favorite bits thus far are the The Unicorn: A Mythological Investigation and the entire damn Judaica section. If you’re a special sort of nerd, this site is for you.
But that’s just me. Anyone else? Anyone? Bueller?
http://postsecret.blogspot.com/
New secrets every Sunday. It’s totally compelling and addictive!
The most excellent inappropriately hilarious site for FREE e-cards that I’ve ever seen!
Demotivational website; parodies all of those glurgey “motivational” posters, mugs, etc.
Boards.ie
It’s a giant Irish bulletin board, with really funny threads, and is a great place to find out the news (pretty racist st times though).
Other than here…
di.fm
I can easily survive without the rest.
Great collection of articles, and although some slip by, for the most part they are already vetted as being from reputable publications.
Oh my God, I love you! That site is exactly what I need and never thought to look for!
My contribution: hulu.com. (My actual favorite one would be my local library site, but that would be pretty much useless to the vast majority of posters.)
It passes the time
Please do what the front page advises and buy their DVD. They need support, and it’s too wonderful a service to let die.
You may also be interested in ReligiousTolerance.Org.
http://www.baseball-reference.com/ is my favorite and currently my second favorite is probably The Thain’s Book An encyclopedia of Middle-earth and Numenor.
Astronomy picture of the day. No words for the beauty of the pictures, and the text is informative and usually quite cleverly hyperlinked.
http://uncyclopedia.wikia.com/wiki/Main_Page I can usually rely on the “Did you know” section to give me a chuckle 
http://www.giantitp.com/comics/oots0001.html
My favorite webcomic at the moment.
Really fascinating. Word of advice: If you have any non-cancellable appointments the rest of the day, don’t click.
A game website with badges! Badges are cool.
I really enjoyed TVtropes, but I have mostly exhausted it now.
I love the room escape games.
This bloke posts a slew of links each day. He’s got a special interest in abandoned buildings.
http://www.londonlee.com/chipshop.html
Wicked blog!
I always hear that about this site, but it’s never done much for me…
Joe
The amount of time you must have spent on TVtropes to have exhausted it is truly frightening. I’m teleporting you a couple of hot horny chicks.
Well it does get a little repetitive and I do have a bit of an obsessive personality. Look at me on the Dope.
Count me in the “don’t really get the attraction” crowd re: tvtropes.