This topic is a little tricky because there are a lot lot lot of sites/blogs I find useful/awesome for one reason or another, but here are some particular faves:
Goodreads. This counts, right? It's pretty much pure awesome. I mean, I get to see what other people are reading, and say, "oh yeah! I wanted to read that!" and then put it on my to-read list, and it's just so handy. I'm not the best at keeping up with it all the time, but it's really fun, and I love seeing other people's reviews and such, too.
Absolute Write. Admittedly I've become more of a lurker than a poster, but seriously. I don't think there's a writing related question you can't find the answer to on this forum. There's even a thread about whether you slouch or sit up straight when you write. I'm telling you. Everything.
Melissa Marr's blog. She answers questions about her books, writing, whatever people ask. And it's both interesting and informative.
Kristin Nelson's blog. This was the very first agent blog I ever stumbled across. She covers a wealth of topics, and she doesn't mince words.
Janice Hardy's blog. She blogs so often and it is always interesting and helpful. And sometimes makes me laugh.
The Rejectionist. Purely for the entertainment value. Hilarious.
What are your favorite sites? Share your links at YA Highway.