Client Login Area?

Does anyone know of a way to setup a site with a client login area that has some of the following features:
Database support
Individual client access (for many clients to get in)
owner can update client’s areas using WP
I’m not sure what else I will need to have this do but I am pretty sure someone here will have some suggestsions on setting something up like this. I’m also sure that this is a great topic since so many people that use WP are designers and probably require a similar feature on their site.
So what do you use, or what can you recomend for a seemless integration? I’d like something that is dumbed down that I could potentially also setup for client’s websites as well.
thanks,
Steve