If you do any mathematical calculations, then you might find Maxima and wxMaxima useful.
I searched for a very long time looking for a good CAS (Computer Algebra System) and have found many.
For very sophisticated work, the I think Axiom is the best-designed and most comprehensive system, together with the Aldor programming language. However, this system has a very steep learning curve, and I’ve only scratched the surface of this system.
wxMaxima, on the other hand seems to be a very good compromise between sophistication and ease of use. It has a little command line, but you can compose a command through menus too. It can handle both symbolic and numerical manipulation. It supports
- rational arithmetic
- complex arithmetic
- differentiation and integration
- and so on
It’s easy enough to use as a calculator too, with all the usual scientific functions.