my favourite is pari/gp, that can be used as a simple calculator, but also has quite fancy math at your fingertips. For example, it is the easiest way to comput

my favourite is pari/gp, that can be used as a simple calculator, but also has quite fancy math at your fingertips. For example, it is the easiest way to compute entire series: you type "exp(sin(x))" and you get "1+x+1/2*x^2-1/8*x^4-1/15*x^5 ..."