Simple Affiliate Tracking?

I’m looking for something really – really – basic here.

We’d like to track affiliates who bring us sales. Our sales will most likely be run through a Wufoo form or processed manually offline.

I’ve scoured the plug-in directory, but what I’m finding looks fairly complex (i.e. does too much).

Here’s what I’m looking for:

1. Affiliate sends a visitor to our website and they decide to buy.
2. We get an order – either thru Wufoo or just a simple form capture.
3. We see which affiliate brought us the customer.

In short, we just need to be able to track if/when an affiliate brings us someone who fills out our form to purchase our product.

I don’t need a fancy “back office” or anything like that. I’m simply looking for the ability to track if/when an affiliate brings us a customer.

Do you know of any simple plug-in to accomplish this?