Simple Commerce includes a few popular payment gateways:
If you need to use a payment gateway that's not built-in, you can build your own. There's documentation on doing so in the 'Extended' section.
In development, you may also use the 'Dummy Gateway' which will allow you to enter card details in that'll always be accepted. It may be useful if you're doing some prototyping or you haven't yet decided on which gateway to use.