website building?

Can Ruby on Rails be used to build a website? How good can flash/flex/actionscript files be integrated with a ROR application?

Rails is an application framework for developing web-based applications. It works quite well for building websites and/or web applications. Perhaps you've heard of twitter?

Flex works well with it, too -- Manning has a book, Flexible Rails, about that topic.

Yup, as Matt pointed out Flex with Rails works great. You can use Flash, etc. perfectly fine since it just generates HTML.