This module is based on Ian C. Wilson's Member's discount class. (Thanks Ian).
A simple drop-in module to allow you as admin to specify a different discount percentage for each of your customers. The module can be switched on and off globally via the admin section, and if switched on, only customers with a discount percentage over 0, thus allowing you to switch it on or off per customer.
The only manual change that is necessary is adding a single field to the customers table.