How does something like this sound from a business point of view. I know that access to intranet systems are usually restricted to the particular organisation's own network yet a web based Intranet (basically a website) - Would you say that there will be a market for something like this?

Can you elaborate a bit more on your idea? Almost sounds like you are about to describe Microsoft SharePoint.

most larger companies already have that, an intranet that can be accessed by employees (at least in part) over the internet by logging in through a firewall.

What you are describing sounds like an 'Extranet' which is essentially a website, which a user can login to in order to access additional content that is not available to the public.

Extranets are used widely whenever you have an account, however there is nothing to prevent it (and it has been done many times before) from being used to access business related content. For example a company might have a website, and an employee can login to view the work calendar or meeting agendas whilst a member of the public might login to the same website and view their purchase history.

I also think you mean an extranet, and alot of business already have them, and they are proving to be very sucessful and helpful.