This is a Ruby library for integrating with SagePay. SagePay is a payment gateway for accepting credit card payments through your web app. It offers three different integration modes:
Form, which is the simplest, and just involves crafting an HTML form that submits information to SagePay. This is not something you're going to need much help with, so this gem currently doesn't help you out there.
Server, which is the sweet spot: you can do most of the transaction types that you'd want to do, but you don't have to take/store credit card numbers, so you don't have to worry too much about PCI compliance.
Direct, the full-on integration where you take the credit card numbers directly on your app and the client need never know you're talking to SagePay to do the payment.
The current client app I'm writing is using SagePay Server, so that's where the current implementation will be focused. Direct will follow when I (or somebody else) has the impetus to do so.
You should be able to install the gem directly from Gemcutter, the newly default Rubygems repository. Simply do:
(sudo) gem install sage_pay
and you're good to go.
This gem currently implements SagePay protocol version 2.23. The client app I'm writing is in Rails, so there are probably some assumptions around that, too.