Recently I've undertaken researching various options for accepting credit card payments for Camp NeeKauNis, a non-profit organization I'm heavily involved in. We wanted to be able to accept donations and camp fees through our website, moving from mailed-in paper registrations and cheques to online registration and payment.

On the site side, CiviCRM provides us will all of the functionality we need - all that it needs is a payment processor. CiviCRM already supports a number of processors - PayPal, Google Checkout, Moneris, Authorize.net and more.

Ubercart, a Drupal module designed to add e-commerce to websites, is our de facto choice for adding such a feature. I've investigated the support for these payment processors out-of-the-bod - of course, you can always create your own module to support a new processor.