Ruby + Ruby On Rails ( Twitter, Group On, BaseCamp, Scribd, Hulu, Justin.TV, SlideShare etc..) The Web Framework is designed for Fast coding for Web, really nice design and easy to use.
Python + Django ( Washington Post, http://www.djangosites.org/most-comments/ ) Django is also a Web FW designed to help in fast coding, but as far as I can see it strong point is the admin console which is very rich.
Scala + Lift ( http://lift.poweredsites.org/ ) I am not very familiar with it, I only know that it's a son of Java language, it uses the Java JVM to work, and it's sintaxe looks more like java then the to above.
I myself am learning Ruby because I liked the Web Framework more( I am a Java programmer, java sucks a bit for Web, too complex I think) Python has the appeal to be useful for more stuff like doing linux stuff or getting a job on Google :D
Cheers from Brazil !
PHP totally slipped my mind... but My point was the "New" ones :D I would not advise anyone to learn PHP nowadays... to much trouble for to learn an old language, it's still in use I agree but has too many problems solved by those 3.
Java Web is a pain I work with it and takes a lot of time to master and be able to code anything useful in it... would never recommend it for a web Startup. Ok, maybe for a webbroker I would :D
Twitter has a lot of open-sourced components in Scala, if Rails is still used, its only for the frontend, which talks to these services.
Cheers from Brazil.
Is the top of the list
Cheers.
And yes, I know that you can use JavaScript on the serverside too, but I highly doubt that something like node.js would be the backend engine of the majority of startups. So the way that I'm interpreting the results is that everyone (or most) startups use JavaScript on the clientside, and a majority of startups use Python on the serverside.