nopCommerce site: http://www.nopcommerce.com
Listing on nopCommerce "extensions and themes" catalog: http://www.nopcommerce.com/p/968/worldpay-payment-module.aspx
WorldPay site: http://www.worldpay.com/
Now source code moved to the core of NopCommerce project: : ...Plugins/Nop.Plugin.Payments.Worldpay