I haven't heard of a plugin that does just that but what your wanting is very doable with the WP backend. If you made new signups contributors they would be able to use the backend to submit their posts. If you want people to bypass the whole backend then you will essentially have to recreate the whole post/edit portions of the backend.
Most people that are writing articles online should be familiar enough with the WP backend that they would understand how to use it. In my opinion utilizing the WP backend would be prudent. Trying to recreate it for the sake of keeping a consistent design or user experience would be more work than it's worth.