Whatever tool is best for the job. I'm still using Fortran in some areas of my research (molecular dynamics stuff), but then I use CoffeeScript to post-process

Whatever tool is best for the job. I'm still using Fortran in some areas of my research (molecular dynamics stuff), but then I use CoffeeScript to post-process simulation results. Throw in some C when I need a good balance between speed and rapid development, and Haskell when I'm just messing around ;)