Something Awful has Creative Convention, a general 'creative pursuits' board for posting stuff. Your mileage may vary on how helpful it is, and something like comic script outlines might be difficult to convey to a general audience. (Mostly what I've seen there is writing and art, but some people have posted Youtube videos and other stuff.)
Aside from that, there's not a whole lot aside from if you know people who are also into whatever particular medium you're working in. Usually, those people are the most willing to give frank and helpful feedback, since they've probably got a better sense of what's good/bad in comics compared to, say, your sister.
