I've just released a new module called iTransact::Lite. It's just a simple wrapper around the iTransact XML Connect payment gateway web service.
It does all the hard work of signing, serializing, and submitting your requests to their web service. However, I call it "Lite" because it doesn't do any document formatting or response handling. Therefore it still requires knowledge of the web service documents. Luckily, those are well documented on iTransact's web site.
If you're looking for a new payment provider, you could do worse than iTransact, and with this module, you have fairly easy access to the service. This module is in use in The Game Crafter and our forthcoming Mob Rater service.