When can employee left the organization there are few things that is advised to take care.
1. Access control
It include to access to Email/Data/Office/Remote Access.
Close his/her email account and access to all third party accounts.
Take back the keys which control his/her physical access to organization.
2. Documentation/Knowledge Transfer
Documentation of employee work and projects. Take its backup at secure place.
3. Organization Assets
It includes Laptop/Desktop/Cell phone/Blackberry/Head Phones/Web Cam/key badage/Security Keys/Parking Stickers
Make a checklist of these things and recover them before employee left the organization. This work can be done by IT/HR department.
4. Exit Interview
It is a best idea to ask an employee to fill an exit form which include his view/suggestion about the organization e.g if he has some complains about the company, or want improvement in some process or have suggestion to implement some policy.