passing custom parameters from page to page

How can i pass custom parameters from page to page on drupal?

I would like to keep the affiliate code on all our pages and deliver the code to a third party software which is the affiliate program and shopping cart. I want to clarify that we don't need an affiliate module for drupal

Example:

Our affiliates are promoting our page using their affiliate code like:

www.domain.com/?aff=123

Surfers can navigate for all the site (drupal) but we need to keep the affiliate code "123"
When surfers decide to sign up it will deliver the affiliate code to the third party software like:

www.shopingcart.com/?aff=123

Any idea or module that can help me?