Critters

If you're a budding or experienced author of Science Fiction, Fantasy, or Horror, the best thing you can do for your craft is get yourself on over to Critters and get signed up. Then get a story into the queue and start reviewing.
The world is full of writers. The self-publishing industry is full of writers. Everyone, it seems, has a novel in them somewhere. What sets Critters apart is that is forces participation. If you want to submit a story for review, you have to critique stories that are sent out weekly. You can control what kind and how many come in your email, or you can peruse them all on the web site. As long as you keep your participation level in the 'acceptable' parameters (roughly one critique per week), then when your story comes up in the queue it will go out for review.
I've been "crittering" for two years now, and just sent out my first story last month. I was pleasantly surprised about the number and quality of critiques I received. Granted, some were short and worthless, but a couple were gems where it was clear the author had taken real time to go over my manuscript in detail and make comments. And none were catty, or cruel.
The beauty of Critters is in it's simplicity. Anyone can join, there is no cost. All you must do is keep you participation current to send out stories for review. Critters even has a program in place for review of entire novels. At over one thousand member, Critters is also the largest online writer's workshop I've ever found.
His list of online resources alone is worth visiting the page.
The stories you'll receive for critique vary wildly. Some are woefully bad. Some have little hope. But what I would consider to be a surprising number are either really close or right there, something you would find in a mainstream magazine today. It's hard to be nice to the bad ones (Critters has some helpful hints on diplomatic critiquing). But sometimes it's harder to find negative things to say about the great ones. Occasionally, a grammar-check and personal nit-pick are all they need.
Does Critters work? Well, when I finally apply their advice to my story (I'm letting things 'percolate' for a bit) I'm confident it will be much better. And since members of Critters were Nebula award finalists in 2002, and a Critter'd story won, I think it does.
Critters is the brilliant brainchild of Andrew Burt. "aburt", as he seems to be commonly referred to online, is a professor at the University of Colorado, a Science Fiction author, and much more. His personal page is here.