A Twitter and FB comps section? I don't do FB so can't help out much with that but I understand they have loads of comps going on.
What about an 'I'm Stuck' type section where users can ask for help with a question/quiz or other help in relation to a comp? (such as they haven't received a prize and not sure what to do, etc) - unless that would all be covered in General Chat?
Also ... well you did ask

... for comps that require a purchase - codes required to enter the comps aren't always unique. It would be nice if we could all share our codes in cases like this but could it be private for registered/regular members only to view?
I'm going now before you shout at me. No doubt I'll be back in this thread again
Win American Pie trilogy on blu-ray