I have nothing for C. For other languages, this is a ruby interpreter. But honestly the best online development environment is the browser itself. Javascript is an advanced language. Combined with technologies like CSS and DHTML and frameworks like jQuery or Prototype you can build graphics applications. It is not hard to find debuggers (like Firebug) also.
Of course you can't interact with the file system. To overcome this you could write a plug-in for a browser (notably Firefox). There are many resources available for this and although it is not as straight forward as pure javascript, it is easier than most people believe.
