Well I do know one place, but it's not for posting so much as getting solid feedback. It's been years and year since I participated, but if you want serious reviews and are ready to do some serious reviewing in return you could try http://www.critters.org/.

It's for SF, fantasy, or horror short stories or novel chapters.

Basically you submit your work as part of a queue and once it's your turn you get reviews. But you have to have submit reviews to be eligible to add your piece to the queue. There are lots of newbies but also some pretty serious beginner writers on there. I used it for a while to help me practice critiquing.