Google Checkout Custom Control by Justin Finch (C# Corner) - step-by-step guide on using a custom control that easily integrates your website with Google Checkout.
Build a .NET App for Google Checkout
by Martin Omander (Windowsdevcenter.com) - discusses an open source .NET sample implemention of Google Checkout integration.
Google Checkout Sample Code for .NET
(Google Code) - sample code contains classes and methods designed to construct Google Checkout API requests and handle Google Checkout API responses, notifications and merchant calculation callbacks.