Programming languages

I always get asked by our customers about market share of the major programming languages and what percentage of people are programming in .NET vs. Java or PHP or ... .  Technically .NET should be broken into the specific language as the .NET framework supports multiple languages.  There is no perfect way to come up with these numbers, you can do surveys but the pool you choose tends to be biased, you can check the languages used on websites but that only covers one aspect of applications.  One of the ways that is pretty good at estimating (not necessarily market share but trends) is to use books written on each language (and how many copies are sold). 

Oreilly recently published the state of the computer book market, check out the results for yourself.  It's good to see so much interest in Python (and powershell) recently and also interesting to see the trends of Java, .NET languages and C# in particular.

I'm a big believer in learning more than 1 language and feel that a programmer can pick up a new language pretty quickly (the basics, libraries and the rest will come in time).  It's actually very useful to know multiple languages so that you can choose the right one for each project rather than going with the one you're comfortable with that can be bent to solve the problem.