CC is very handy (shorthand for Critiquecircle). I've been there for close to 2 years, and would highly recommend it especially for beginning writers. Of all the different sites I've seen, the system is by far the best and most user-efficient.
In case you don't know about it yet, a very handy function is to ask for predominatly Inline critiques. Then, after the period, clicking on your story you'll see a box with the list of all your crits and also an option that allows you to bring up all of your critiques on one page. So, instead of having to read through every different critique, you can get to see them all at once.
Speeds up revisions a lot.
My username there is Davidbeer, and I'm there most days. Anyone who comes to the site new and needs a hand, you're more than welcome to drop me a mail and I can see if I can help you out.
There's also Critters:
www.critters.org
and it does work for a lot of people, but I'm not keen on the system myself.
Would rather recommend Liberty Hally:
www.libertyhallwriters.org. It's mostly for shorts, but I know several writers who are there, or used to be, and quite a few of them have been making some very good sales and critical progress since then.
They also use various triggers to help writers get writing.
There's also OWW,
http://sff.onlinewritingworkshop.com/, a paid workshop unfortunately, but again highly recommended for Fantasy and SF writers.
some good news in this regard, writer Joshua Palmatier is holding a competition where participants can win memberships for free in OWW.
Details here:
http://jpsorrow.livejournal.com/127997.html
Finally, there's Codex:
www.codexwriters.com
This one, though, is for neo-pros; in other words, writers who've been writing long enough and succesfully enough to no longer be newbies, but are not quite at pro level yet, at least not consistently. Mind you, a few of the people there do have succesful book sales behind them.
For newbies (however you want to define that, whether in writing terms or success rates), I'd recommend CC, Liberty Hall first and consider one term in OWW.
Down the road, when you get some progress going, Codex is a great place to take you to the next level.
It depends on what you're after, and whether it's for shorts or novels you want to focus on, but all those places work for various people.