For large scale web applications most of these frameworks will not work because they don't scale. I know Twitter is an exceptional case but why do you think they are converting their Ruby code to Scala?

I personally like Grails which uses Groovy for the programming language. To enhance performance and obtain near Java like speed there is a statically typed extension called Groovy++. Because it operates heavily off of type inference it is extremely non-intrusive when being introduced to existing code.