Ordinarily I'd recommend AbsoluteWrite, but they have a strong preference for authors leaving critiqued work posted so others will learn both how to write better and how to critique.
There's no reason not to leave your work posted if the site is not accessible to internet 'spiders' and searchers because it's protected by a password, as AbsoluteWrite is. The theft of writing by others is relatively rare, and theft resulting in a sale almost unheard of.
I've found the critique there to surpass what I've been able to get elsewhere, except for my in-person writing group.
