You don't really specify what you want to achieve -- a "website" can be many things. If you know a little Ruby and like working with it, I don't see why you wouldn't go with RoR.
If you want to stay a little closer to Java, you might want to check out Grails:
http://www.grails.org/
https://devcenter.heroku.com/articles/grails
If you're actually just writing a website, not a web app (however that distinction is made these days), something like Ruby and Sinatra might be a better fit:
http://www.sinatrarb.com/
https://devcenter.heroku.com/articles/rack
And if you want something completely different, you might have a look at web2py (on Heroku you would have to deploy that as a python wsgi app).
And while I think Heroku looks fantastic, I'm personally leaning towards http://appfog.com/ for the projects I plan to be toying with "in the cloud" -- I feel their pricing model (especially their "free" tier) makes more sense. But you should of course choose what is a good fit for your needs.