SafeCharge (formerly Gate2Shop) is a payment solution for small to mid-sized enterprise. PCI compliant API makes it easy (for a developer like me) to integrate.

Stripe is the new kid on the block in terms of payment solutions.  Because of this, they have been able to leap-frog over past technology to offer some of the newest tech in payment solutions.

CashFlows is an online payment gateway for small to mid-sized companies.  With PCI compliant integration, Cashflows offers a clean, safe API for allowing your customers to pay with credit cards.

Most websites end up needing some form of Payment Solution.  PayPal, the grandfather of online payment solutions, is a great choice for small groups and businesses.  There are many ways to integrate PayPal into your site to get the money flowing.