HOW TO: Integrate the PayPal Payment System with ASP.NET

Integrating PayPal Payments into E-Commerce Applications with ASP.NET by Rick Strahl (West Wind Technologies) - describes how to minimize external PayPal interaction and work the PayPal payment into the order processing workflow to provide a seamless interface using ASP.NET and C#.
Use of the PayPal payment system in ASP.NET by Alexandr Golovanov (CodeProject) - covers aspects of using the PayPal payment system (e.g. One-click shopping, IPN processing) in ASP.NET.
Creating a simple paypal shop (ASPCode.net) - this sample implementation uses simple redirect to PayPal.
ASP.NET: How to Integrate Both Google Checkout and PayPal In 3 Steps by Andreas Viklund (.NET Discussion)
Related: ASP.NET, PayPal, and Google Checkout: The Solution!
Create a PayPal Instant Payment Notification (IPN) Handler (wwwcoder)