Here is what I have in my professional toolkit. These are things that I always try to have on my system. Everything here is free with many them being open source. I originally published this list about 15 years ago and this year I finally revisited it and cleaned things up. What I discovered was that the open source projects are the ones that most likely survived the test of time.
Go Programming Language - Open source programming language that makes it easy to build simple, reliable and efficient software (Quoted from the Go site but is pretty accurate). This is my favorite language as it blends the compile nature of C with the practical aspects of a scripting language.
GNU GCC - Open source C/C++ compiler supported on so many platforms now that I can’t list them on. There’s even ports of it so that you can compile games to run on the GBA. C is the foundation language of enabled the growth of our profession and is a must learn of every software engineering.
Perl - One of the great scripting languages although admittedly I rarely use it now. In many instances, Python has taken the mantel of Perl, but Go might replace both.
Scheme - I don’t really do much with Scheme, but I believe it’s worthwhile for engineers to have exposure to this form of programming.
Vim - Although I started as an EMACs user, VIM is now my primary editor..
GNU EMACS - EMACS is more then an editor, it’s a way of life!