Here is a list of Customer Tracking solutions, controls, and extensions that work with PayPal:

AceFlex B2C
Back Office Sales Force Management solution for Small Business Owners

JoiHost
PayPal IPN Logging and Notification System.

LeverUp

Scroopers Order Central

TrackPay
TrackPay.com provides real-time customer and product tracking for PayPal Website Payments, PayPal Shopping Cart, and Send Money requests. TrackPay also provides affiliate tracking and registration services