It's hard to gauge what's considered “most popular”, because that depends on which circles you move in. However, I'll list some common ones, and leave you to pick one.
“Web programming” is very broad: you can be writing code for client-side web applications (such as Java applets, Flash, Ajax components, etc.), or server-side web applications (incorporating business logic for web transactions, usually via a framework; see below).
So, in order to answer your question, you need to decide which sphere of the web programming world you wish to play in.
After you've decided, here are a few choices of languages.
Java applets: Usually written in Java; can also be written in JavaScript (via scriptlets [1]).
Flash applets: ActionScript.
Ajax components: JavaScript (by definition).
Server-side web programming is usually done in a framework. Each framework supports its own choice of language. Examples include (see [2] for more):
Ruby on Rails: Ruby (by definition).
Phobos: JavaScript.
Grails: Groovy.
Struts: Java.
CakePHP: PHP.
You can also do server-side web programming without a framework, in which case you have a wide choice of languages: Java, PHP, Ruby, Perl, JavaScript, etc.
(Disclosure of bias: I'm a Java developer, so many of the topics mentioned do tilt in that direction. In particular, I have not made any mentions of any .net-related frameworks, for I don't do any work with them.)