Online Payment ProcessingDiscussions concerning topics related to payment processing, such as merchant accounts, gateways, credit card processing options, PayPal, Google Checkout, 2Checkout, etc.
Paypal is the best option. Though your customers will have to register first but that is just a small form.
Your customers DO NOT have to register with PayPal to pay for things.
Quote:
...For new buyers, signing up for a PayPal account is now optional. This means your customers can complete their payments first, and then decide whether to save their information in a PayPal account for future purchases. The checkout is more convenient, resulting in more completed sales and satisfied customers....