Login for guests and members plugin?

Hi,

Apologies if I’m not posting this in the right category.
I’m pretty crossed eyed by know reading and searching for a little plug-in.
Now I’m not sure anymore if it exists.
What I would like is to have a login and register possibility in my side bar (I think this is what it’s called. I’m using the borderlinechaos theme)
The whole attention is that only registered members can post edit and such. Guests can only read and nothing more.
Is there such a thing and if so could you pls point me to the right direction in where I can find such a plug-in?

Much obliged CaramellaMorbide.