I've noticed that although the blogosphere is filled with writers eagerly learning their craft, almost none of them seem to be taking advantage of sites like Authonomy (link is to my book there). I've been a member since December 2009, and it's one of the most useful sites I have joined. It leans slightly toward the British side of things, since it is a Harper Collins site, but there are writers from all over the world there. I've recently been getting some great feedback for my book from a wonderful Belorussian writer.
As with any such site, one must have a bit of a thick skin to be able to ignore the few people who are not diplomatic enough, but most there are decent critiquers and pretty supportive. You just need to filter out the bad and concern yourself with the good.
The first time I posted a book there I had already completed it. Not many take the time to read the entire novel, which I can understand since reading on a monitor isn't the most pleasant experience, but a few do, and most will read some of it and generally leave some useful feedback. I have made thousands of corrections on my books due to Authonomy critters.
I've been posting my latest book chapter by chapter as I write it, and I've found this very useful. The critters have pointed out a number of things that have saved me lots of further effort down the line if I had waited to post only after completing the book. People seem to be enjoying this book far more than my first, perhaps because thrillers are more popular than Tolkienesque fantasy, but also perhaps because I am a slightly better writer the second time around.
I would never ask anyone to support me blindly, but if anyone finds Authonomy useful and checks out my book, I can always use more support. BTW, it actually doesn't help a book if you support it immediately upon joining. You need to add some books that don't mean much to you (or other ones you find that are good) and first get a ranking before adding the books you truly like to your shelf. Only with a ranking will books accrue benefit from your support. The longer you leave them on your shelf, the more points they gain.
After a year and a half, my first book reached around 191 in the rankings (out of tens of thousands of books on there). My second book has reached 84 after just over one month! If you like thrillers and/or science fiction, perhaps check it out!