back-end languages and front-end languages

back-end languages (in order of my ability to recall):

• Java (and other JVM languages like Scala, Groovy, Clojure)
• PHP
• .NET (C#, VB)
• Ruby
• Python
• Perl
• Javascript  (Node JS)
• Actionscript (Flash Media Server)
• CoffeeScript
• C (CGI)
• Erlang
• oh, and SQL for db queries

For browser-based front-end languages, you're somewhat limited in what the browser can support (excluding launching out-of-browser applications). We could talk about:

• HTML
• Javascript
• CSS
• Actionscript
• CoffeeScript (compiled to Javascript)
• XML-based languages (X3D, SMIL, SVG, DITA, some interpreted by the browser, others transformed using XSL)
• VBScript
• Silverlight
• Java (applets)

For native PC desktop front-ends, most popular front-end languages would probably be (I'm guessing, in no order): 

• Visual Basic 6 (from my experience with big enterprises, I bet a lot of those are still out there, just like Windows Vista)
• .NET
• Java (Swing apps)

But just about every language that can create desktop apps also has some net code library that lets you write n-tiered applications (Kivy (Python), Tcl/Tk, ...).

For native mobile application front-ends, most popular would probably be: 

• Objective C (for iOS apps)
• Java (Android apps)

But if you just want most popular (browser-based) web application development stacks, that would probably some combination of the following:

Front-end: HTML / Javascript / CSS
Back-end: PHP / Java / Javascript / Ruby / .NET / Python