Next language to learn?

I've been coding in .NET for a while (ASP.NET MVC). I like the direction MS is going with Open Source, but I want to branch out into some LINUX type stuff too. I have an idea for a site that I could code up in about a month if I used .NET.

But I want to take the opportunity to kill two birds with one stone, and build it using all LINUX. So what's the best framework for me to learn? Ruby, Node, PHP? Which skills would be idea for freelancing (planning to leave curr. employer by end of year). I'm not too keen on Ruby, the smug Ruby developers I've known have turned me off to it, but I realize that's kind of a stupid anecdote to carry with me too.