Centralizing the Authentication Process

Hello Friends,
Our department has created a few (6) Web Applications and all these
applications share a common piece of code which does the "User
Authentication".

I would like to isolate the authentication process.
I am thinking of having a common/central server which gets all
authentication requests and creates a valid session for all users who have
successfuly logged in.
If an unauthenticated user tires to access these application then he/she
should be redirected to the login page on this central server.

This will help me reduce the time required for maintenance and deployment.

How should i go about achieveing this. Is there an article or any
documentation on this
Is there a server which could help me achieve this.

Any help/guidance will be greatly appreciated.

Thanks,
Mohit