PayPal's Website Payments Standard is an awesome way for businesses to accept credit card payments with minimal integration & no need to worry about credit card security and PCI compliance. Plus, if you offer a SaaS (Software As A Service) as we do at Church Office Online, PayPal will handle your subscriptions & recurring payments for you. Plus they give you a very friendly interface to manage your subscription, issue refunds and view reports for all of your activity.
My Implementation
Even though the PayPal Website Standard integration is very straight forward, I decided to go ahead and create an ASP.NET implementation to wrap up the HTML variables that they accept in their URL querystring so I wouldn't have to create a button for everything. This implementation make customizing the integration very straight forward and gives you flexibility in what you want to invoice through PayPal.
Download
I posted a detailed overview and the download link on my personal blog a while ago. You can find it here:
http://www.schnieds.com/2008/01/aspnet-implementation-for-paypal.html
-Aaron