The Skype client for Windows is written in Delphi. So now that Microsoft owns and operates Skype, they are now customers of Embarcadero, owner of Delphi, the world's greatest software development tool.
Of course, given the corporate culture at Microsoft, I expect the rewrite to 100% ".net" to take them about 1 year. As of November 2012, I will probably have to amend my statement, to say that, from its inception until recently, Skype for windows was written in Delphi. Now it's written in .... something else.