Is there an application which supports a Mathematica-style workbook but customisable?

Question

My son demonstrated Mathematica to me a while back. Is there anything similar which works within the workbook metaphor, but which is not limited to Mathematica?

Answer 1

Seco (formerly Scriba) supports many languages in Mathematica-inspired notebook style.

Note that, with a bit of programming, Mathematica notebooks are not limited to the Mathematica language.