Create a post in current category?

I would like the ability on my site for visitors to create posts in the current category they are in on their current page. I want to do this on the page, not in the backend. To see EXACTLY what I mean, just go to any section on craigslist and click post ad. It automatically knows the category you are in and creates the post in that category, on a page, not the backend. Any plugins that do this?