Reddit - If you're not a Redditor yet, please become one. It's the best site on the internet. You can choose subreddits that are tailored to your interests and become of part of worldwide communities that sometimes do more than share cat pictures.
Goodreads - It's the IMDB of books and a great place to see how your friends and enemies enjoy or hate what they're reading.
Letterboxd - For movie lovers only. Rate films, and write reviews if you want, and connect with others who do the same. This site is a must if you want to go all Siskel & Ebert on someone who disagrees with your take on movies.
Stumbleupon - Great for taking writing breaks, not only to feed your brain ideas, but to give it a rest from your story as well.
Visual Thesaurus - The best investment of $20/year a writer can make.
Script Notes - My favorite podcast.
Grammar Girl - If you're a writer, force yourself to visit this site every now and then, even though Grammar is so freaking boring.
IMDb Top 250 - See where your favorite movie ranks among the top loved films of all time.
JPL Images - See what's actually out there.
Nick Bostrom - Love reading this guy's papers.