PHP is the most widespread. It misses a lot of what I consider to be core functionality for anything beyond simple sites - but you really can get stuff up and running in no time with it.
A more heavily software engineered site would typically be done in Java or .NET; although sometimes I find both of those languages to be a little heavyweight.
Ruby and Python are both well suited to the task, Ruby was the hot new thing a few years back; but I think Python has more Inertia (and certainly more developers know it).
One of the original serverside scripting languages that died a death is making something of a resurgence, Javascript. I never quite got why there were seperate back end and front end languages and with Google throwing it's weight behind stuff like V8 it's interesting.
Whatever language you pick, you'll likely want to pick up some sort of framework along with it - so you don't have to reinvent too many wheels.
Here's a starter list of stuff to look at, it's not by any means exhaustive - but there's a good wikipedia article to list some options...
PHP - Zend, Code Igniter, Symfony
Python - Django
Ruby - Rails
Java - Grails, Spring
.NET - No Idea
Javascript - Maybe Node.js (not really a framework per se.)
For your purposes, i'd probably pick Python + Django. It's easy to learn, and teach you some relatively good habits