1. PHP:
What it is?
It is an open-source, interpretive, server-side, cross-platform, HTML scripting language, especially well-suited for Web development as it can be embedded into HTML pages.
Why you should learn it?
It’s particularly widely used. High-speed scripting with caching, augmented with compiled code plug-ins (such as can be done with Perl and PHP) is where the future is. Building Web apps from scratch using C or COBOL are going the way of the dinosaur.
2. C#
What it is?
A general-purpose, compiled, object-oriented programming language developed by Microsoft as part of its .NET initiative, it evolved from C and C++
Why you should learn it?
It’s an essential part of the .Net framework. Learning C#, which is just Java with a different name plate, is critical if you heavily use Microsoft
3. AJAX (Asynchronous JavaScript and XML)
What it is?
Though technically not a programming language, AJAX uses XHTML or HTML, JavaScript and to create interactive Web applications.
Why you should learn it?
Ever since Google Maps put AJAX, well, on the map, the requests for AJAX-knowledgeable pros went through the roof. The demand for AJAX knowledge is huge because it’s so damned hard to learn. Of note, Microsoft announced recently plans to release a tool named Atlas that will make AJAX easier to implement. If Microsoft’s Atlas tool is successful, it would bring the extreme complexity and annoyance of AJAX to the average worker.
4. JavaScript
What it is?
Not to be confused with Java, JavaScript is an object-oriented, scripting programming language that runs in the Web browser on the client side. It’s smaller than Java, with a simplified set of commands, easier to code and doesn’t have to be compiled.
Why you should learn it?
Embedded into HTML, it’s used in millions of Web pages to validate forms, create cookies, detect browsers and improve the design. With its simplicity to learn as well as wide use, it’s considered a great bang for your educational buck.
5. Perl
What it is?
Perl is an open-source, cross-platform, server-side interpretive programming language used extensively to process text through CGI programs.
Why you should learn it?
Perl’s power in processing of piles of text has made it very popular and widely used to write Web server programs for a range of tasks. Learning some form of scripting language, such as Perl or PHP is critical if you are doing Web apps.
6. C
What it is?
It is a standardized, general-purpose programming language, its one of the most pervasive languages and the basis for several others (such as C++).
Why you should learn it?
Learning C is crucial. Once you learn C, making the jump to Java or C# is fairly easy, because a lot of the syntax is common. Also, a lot of C syntax is used in scripting languages. You can get more C related questions in my post C language FAQs.
7. Ruby and Ruby on Rails
What they are?
Ruby is a dynamic, object-oriented, open-source programming language; Ruby on Rails is an open-source Web application framework written in Ruby that closely follows the MVC (Model-View-Controller) architecture.
Why you should learn it?
With a focus on simplicity, productivity and letting the computers do the work, in a few years, its usage has spread quickly. As a bonus, many find it easy to learn.
8. Java
What it is?
An object-oriented programming language developed by James Gosling and colleagues at Sun Microsystems in the early 1990s.
Why you should learn it?
Hailed by many developers as a "beautiful" language, it is central to the non-.Net programming experience. Learning Java is critical if you are non-Microsoft. Java have many versions as per application needs. The more details are given in article Difference between Java versions
9. Python
What it is?
It’s an interpreted, dynamically object-oriented, open-source programming language that utilizes automatic memory management.
Why you should learn it?
Designed to be a highly readable, minimalist language, many say it has a sense of humor (spam and eggs, rather than food and bar); Python is used extensively by Google as well as in academia because of its syntactic simplicity.
10. VB.Net (Visual Basic .Net)
What it is?
An object-oriented language implemented on Microsoft’s .Net framework.
Why you should learn it?
Most argue that VB.Net is currently more popular than ever and one of the only "must-learn." It is currently dominating in adoption and that is where all the work is.