[i-blis]

J is, within the APL family of languages, the one I have always found the most useful. I reach for it as I used to reach for my HP calculator in the past, to quickly compute or numerically model things. I always have it running in a terminal window.

Contrary to popular belief, its learning curve isn't steep. I once introduced it to high school freshmen who had no real experience with programming. I recommend the series of booklets by Kenneth Iverson himself: Arithmetic, Algebra, Calculus — there is even a Concrete Mathematics companion to accompany the book of the same name by Graham, Knuth et alii. They're all available for download on the site.