Okay, so I couldn't choose just one, so here's a few of favorites that I visit pretty regularly.
I use Biblos just about everyday, it's how I do my studying in the Bible, and I love being able to study it in different languages.
I also enjoy Pandora, I like listening to the radio, but it seems there's always stuff that's questionable, I like Pandora because I can choose what artists and I want to listen and if one comes up that's a problem I can block them.
Goodreads is another of my favorites, I mean come it's a reading and book website, how I could I not love it! I also use it to find and collect quotes, so much easier than jotting them down in a notebook, though I do that a lot too.
And here's just a few fun ones that I enjoy checking on every once in a while.