From Shopify Wiki

What methods of payment can my customers use?

Shopify integrates with many different payment processing services, and more are being added all the time. Just give Shopify the details once you’ve set up an account with one of them:

For the up to date list, please click here: http://activemerchant.rubyforge.org/

Direct Payment Processors

• Authorize.Net CIM - US
• Authorize.Net - US
• Beanstream.com - CA
• Braintree - US
• CardStream - GB
• CyberSource - US
• DataCash - GB
• Efsnet - US
• eWAY - AU
• E-xact - CA, US
• InstaPAY - US
• LinkPoint - US
• Modern Payments - US
• NELiX TransaX - US
• NetRegistry - AU
• NETbilling - US
• Ogone - Europe
• PayJunction - US
• PaySecure - AU
• PayPal Express Checkout - US, CA, SG, AU
• PayPal Payflow Pro - US, CA, SG, AU
• PayPal Website Payments Pro (UK) - GB
• PaymentExpress - AU, MY, NZ, SG, ZA, GB, US
• PayPal Website Payments Pro (CA) - CA
• PayPal Express Checkout - US
• PayPal Website Payments Pro (US) - US
• Plug'n Pay - US
• Protx - GB
• Psigate - CA
• PSL Payment Solutions - GB
• Quickpay - DK
• Realex - IE, GB
• Sage Payment Solutions - US, CA
• SecurePay - AU
• SecurePay - US
• SecurePayTech - NZ
• SkipJack - US, CA
• TransFirst - US
• TrustCommerce - US
• USA ePay - US
• Verifi - US
• ViaKLIX - US
• Wirecard - DE

Off site Payment Processors

• PayPal Website Payments Standard
• Chronopay
• Nochex
• Banca Sella GestPay
• 2 Checkout
• HiTRUST

Checkout Replacements

• Google Checkout