I have developed an online based service where people can help market their businesses using my platform. I have spent many hours developing the core functionality, but have totally skimped on the login/credit card processing/website layout etc.
Is there a good framework out there that I can easily slot into, without having to develop a lot of this "generic" stuff from scratch? The service is made in ruby, but it can easily slot into any language necessary due to my design.
It highly depends on how flexible your 'core functionality' is, and the degree of flexbility you built into it.
The usual suspects of open source CMS systems comes to mind, if your application mostly requires standard functionality, and you are able to build modules for it, to match your specific requirements. If you can't find any that works for you, you go to the programming framework of choice and develop one yourself.
You need to decide how many components you want to keep 'in-house'. You can outsource most of your websites core functionality (if you choose to do). I like Amazon DevPay for creditcard procession, and for account handling you can check the possibilities at Stack Overflows login.
But ultimately, you will not get a straight answer on how to build your website.