Critters is cool and it's the only online workshop I've been using. But not every story I write falls into the genres they crit. So I found another site (Critique Circle)that will crit just about every genre.
They work on a credit system. It seems pretty interesting. I haven't joined yet, but I'm considering it. It's free to the best of my knowledge. Details available on the newbie page I've cited.
Here's the link if you're interested: