If you work professionally or for a hobby in the world of mathemeatics, then Scilab is an interesting tool. It could become essential, especially for working on numerical calculations.
Scilab has a very intuitive and friendly interface and setting. It lets you do all classes of mathematical operations: create data structures (polynomic and rational matrices, of text, lists, multivariable linear systems, etc), hundreds of already predesigned mathematical functions, graphic expressions in 2D and 3D with function behaviour, already developed libraries ready for use, linear algebra (including dispersed matrices, Kronecker forms, Schur ordering, etc).
Others include signal behaviour study, linear matrix inequations, optimization (differentiation and non-differentiation, LQ solutions), metanet, etc.