Developement of embeddedd software (aka firmware) for microcontrollers of all kind, with or without operating system, generic or real time:

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• C
  best programming language to get small footprint and efficient firmware.
  C is best used on microcontrollers with limited resources and if the software doesn't get too big.

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• C++
  C++ is a higher programming language. It's concept is to represent the "world" by objects. It is used for systems requiring larger software which isn't well covered with a procedural approach like with C.
  Still C++ has a really good performance (unlike Java e.g.).
   

• Assembly
  This one is used to meet hard real time requirements where the duration of executed tasks need to be precisely known. Actually it's the native language of each microcontroller/processor but programming in assembly increases developement time exponentially. So it is only used when really needed.
   

• VHDL for FPGAs
  Well, this isn't a programming language, it's a Hardware Description Language (HDL)! FPGAs aren't processors or microcontrollers. They do have a a gigantic array of basic logic blocks (Gate Arrays) which you build up (i.e. describe), configure, to a higher purposes with that HDL code. Of course one can build his own micrcontroller that way - called a softcore processor - but you still have to write its software - in assembly, C, C++ - to run on top of it. FPGAs are mostly used in systems processing high volume and fast data streams where microncontrollers are to slow because of their sequential nature. Tipycal applications are professional video equipement, networking gear, algorythms, TVs and much more.