Maybe this has been brought up here already but I just stumbled across Issuu.com
At a glance it appears that full publications are online here (in a nice format too). Publishers are uploading stuff themselves but I haven't yet determined if any of it is simply user-uploaded (which, of course, wouldn't be particularly cool). But there's a lot of interesting stuff to browse through.
Here's a bit more info over at the Zine Arcade site: