PayPal.Net
A recent Weblogs.asp.net post heralded the release of PayPal's new ASP.Net SDK. They also expose many of their features as web services.
A clever SharePoint developer might create a web part or series of web parts that implement the ASP.Net web controls in the PayPal ASP.Net SDK. Also, consuming the PayPal web services using the data view web part is fairly interesting. Some of the services don't make sense being consumed by the data view web part, but it ends up looking much more professional than the PayPal button.