With node and all its frameworks starting up, will javascript be the next big thing, or will it still be rails or is there some other technology popping up?
And if you're wondering this because you're trying to decide whether to learn Javascript, the answer is Yes. Learn Javascript, because wherever the trends go, knowledge of Javascript is still crucial for web development.
* Ruby is popular, but Php is still the most used and now there's Laravel - that's a big thing in the php community - you'll like it, if you work with rails. Not sure why you mentioned php, python and then..rails :T
Use whatever you like and create the next big thing.
A few other big things... - MeteorJs - PhantomJs - Your idea here, using whatever you like ?
Obviously, though, any web developer is going to need to know Javascript. On the client side, use of Javascript frameworks like Ember/Angular are clearly the future.
If javascript would be next big thing then surely can't reach there without HTML5.