Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
PayPal - PayPal is the faster, safer way to pay online without sharing financial details, send and receive money or accept credit and debit cards as a seller
Anchor CMS - Free and lightweight blogging system
Stripe - Online payment processing for internet businesses. Stripe is a suite of payment APIs that powers commerce for online businesses of all sizes. Use Stripe’s payment platform to accept and process payments online for easy-to-use commerce solutions.
Textpattern - Textpattern is an elegant content management system that is free, open source software.
Braintree - An all-in-one solution to accept, process, and split payments in your mobile app or online - from small business to large enterprise.