Frink is a mainstay on my phone, despite its not being open source - I make an exception because it's so darn useful. I don't use its programming features, though - it's kind of hard to be bothered with a weird language on a phone, even if unit-awareness is a great feature.

On desktop, for calculation and unit-conversion, I've completely replaced it with Qalculate, which doesn't have a programming language but does have a symbolic solver built-in, which is surprisingly useful for general calculations: you don't have to rearrange your calculation so the unknown is by itself, you can just write the equation from your textbook and replace the thing you want to know with 'x'.

Frink's "Sample Calculations" segment is an amusing read, in much the same ticklish way as xkcd's "What-if" series.