On NetTuts.com there's a tutorial showing you how to use the GoCardless service to accept payments on your site (in the UK, unlike some other online payment services).

Online payments are increasingly becoming more and more popular. GoCardless is yet another payment service, which currently works in the UK, allowing you to accept payments online.

Most of the tutorial is used to explain what the service is, getting you set up with an account (complete with screenshots) and how you can work with their API via an SSL connection. Included are examples of this using their PHP Library and what the results look like in their interface.