Paypal
Paypal's strength is the fact that it's a common means of transferring money online (as any eBay user will tell you). The service is free to join, with fees calculated as a percentage of the transaction amount received. What makes Paypal so good is the fact that buyers don't need an account to make a payment -- they can simply use their credit card. Paypal offers a wide range of free shopping cart features, and it is catered for by almost all shopping cart software (osCommerce, etc).
