Knowing some C/C++ is very useful if you want to get a better understanding of what the computer is doing. Or Assembly of course, but that gets complicated fast

Knowing some C/C++ is very useful if you want to get a better understanding of what the computer is doing. Or Assembly of course, but that gets complicated fast. Common Lisp / Clojure / Scheme (or any language from that family) will really let you appreciate some fundamental functional programming tricks once you finally "grok" them. Prolog, it's great. You'll never look at graphs, trees, search or first order logic the same way. And, maybe, some day you'll be the one that says "that's just two lines of prolog". Haskell, or any other very strongly statically typed language. I'm definitely partial to dynamically typed languages, but you gotta try it before you knock it :-) Also if you have some money to spend, try Mathematica. It's one of those things that keeps getting reinvented, but poorly. Shamefully it's not open.