After years working with telcos and then in the network monitoring field, I acquired a large experience in system programming in the Unix (or Linux) environment, for designing and implementing realtime software, networking services or tools. C/C++, GNU toolchains, git, etc.
If time and required performances requires it I can also work in assembly or, at the contrary, use a higher level language such as OCaml or Haskell, or a byte code based language or a script language, to fit your environment.