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.
Wirize - Payment checkout for freelancers
Google Payment API - Enable an easy-to-use checkout experience for customers
Giftbit Payment API - Stripe for gift cards
Pay Me Or Else! - How to avoid and deal with clients who don’t pay (ebook)
ACH payments with Stripe - Accept bank payments with a few lines of code.