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That depends on what you're trying to do. LAMP (Linux / Apache / MySql / Python) is a tremendous platform for web development, especially amongst organizations that are trying to avoid paying Microsoft fees for servers, development tools, etc. But many of the web developers I know are more interested in Java + Ruby for making web apps.
Apple's IOS platform (for iPhones, iPads, and the iPod Touch) is currently the one with the most worldwide demand. Initially IOS required coders to use Objective C (or Objective C++) but as of September 2010 Apple opened up the platform to use interpreted languages, as long as they don't download and run other code than what's in the bundled App. XCode is a good programming environment, but there are even ways to use .NET tools for building IOS apps.