Manga Helpers: These guys offer scanlations of comics that aren't available in the U.S., and it even has forums where you can talk with other fans.
ListenToYouTube.com: If you like a song or need a sound clip from something on YouTube, you can translate it into an mp3 you can download here.
TV Tropes: Chronicles the many patterns and clichés of fiction of all types (despite the "TV" reference in the name). So addictive that in some places on the Internet you're required to warn people if you're posting a TV Tropes link since it often results in the loss of hours.
FOOD:
Chef2Chef: A chef's practical guide to practically everything.
Supercook.com: Find recipes that use ingredients you have at home!
The Cook's Thesaurus: A cooking encyclopedia that covers thousands of ingredients and kitchen tools. Entries include pictures, descriptions, synonyms, pronunciations, and suggested substitutions.
Urban Dictionary: You can find out what all those ridiculous things people say mean if you're just too out of touch to know every single piece of slang on the planet.
LITERATURE:
What Should I Read Next?: A cross-reference database to find new things you like based on things you read.
All Our Worlds: A searchable database of science fiction and fantasy that allows you to search for diverse and underrepresented elements. Want SF featuring characters with disabilities? Want to read fantasy with gender fluid protagonists? Want an all-female cast? Got you covered!
Goodreads: You can join, make a profile, rate and review your favorite books, and get a friends list going so you can have your friends' reading habits and recommendations mailed to you!
Graze: For five bucks a week, Graze delivers boxes of four different snacks to your mailbox. You can rate the snacks on their website, and the ones you rate most highly you will receive more often, while if you get something you hate, you can rate it in a way that makes them not send it anymore. Great for work!
Wikipedia.org: The best research site to get lost in ever. . . .
The Wayback Machine: Use this site to type in any URL and find out what it used to look like at any particular time. This is especially useful if a dead page used to have great content and you can't get it anymore; it might be archived here and saved after all!
The Average Woman from Each Country: Through blended photos of thousands of women, the photographer comes up with a basic "average" of what a woman from each featured country looks like.
SKEPTICISM/SCIENCE:
Skeptic's Annotated Bible: Notes for understanding the Bible, mainly to point out contradictions and absurdities.
Pharyngula: Some of the best biologically oriented and snarkily skeptic articles around. This guy is quite a joy to read.
Skepchick.org: A nice blend of science, skepticism, and feminism that I love to read.
RichardDawkins.net: This English biologist has contributed much to science.
SamHarris.org: I appreciate Sam Harris's views on faith and religion when balanced with science.
Limyaael's Fantasy Rants: Fantasy writers (and any kind of writer in some general sense) will need to avoid doing some of these things that annoy Limyaael. Because if you avoid these things, you will be a much better writer.
Authonomy: Run by HarperCollins, this is a site where serious authors can post fiction--preferably whole novels--and, through a rating system and promotion and whatnot, can end up getting it read by an editor at this major publisher. Skip the slush pile!
Duotrope's Digest: A database of thousands of current markets for short fiction, poetry, and novels/collections.
The (Submission) Grinder: Search for markets for your work, filtering for word count, genre, story subject, pay rate, and more!
Absolute Write forums: Talk with other writers about publishing, story ideas, critique, or agent searches. It has a great "bewares" section to avoid scams.
Preditors & Editors: Look up writing-industry professionals to make sure they're not scam artists.