I am working on a software-as-a-service (SaaS) application and I am looking for a billing plugin of some sort that will manage my subscriptions, customers, and recurring billing. There is the RailsKits SaaS kit ($249.00), but I prefer to use open source software. I have also found maccman's saasy, but the phrase "At the moment this is alpha code - use at your own risk" makes me a tad bit nervous.
You could just outsource the whole SaaS side of things.
http://Recurly.com
http://chargify.com
http://spreedly.com
http://cheddargetter.com
http://fusebill.com
http://chargebee.com
There are other providers like SAASY.com, 2Checkout.com who provide a bundled merchant account + payment gateway service along with subscription billing.