I can't believe to read "the power of JavaScript" (but I had to upvote the answer for the other parts, except perl of course.
Practically, for the simple cases where integer arithmetic is sufficient, I use the buildin $((...)) and recommend it. Else, in almost all cases echo "..." | bc is sufficient.
For some arithmetic operations like statistics, matrix operations and more R is the better tool:
echo 25 + 5 | R --vanilla
and for small datasets and graphical throw away results, oocalc is a nice utility.
