Bob's Calculator is a simple command-line calculator. Calc is a programmable calculator that uses a C-like language.
E is a a purely equational theorem prover for clausal logic.
Genius is a calculator program similiar in some aspects tobc and matlab. GMatH is a mathematical computing environment. It should soon include all NumPy functions and online helpers, plus basic plotting functions. The GNU plotutils package contains software for both programmers and technically-oriented users.