Yacas is described as 'Yet another (general-purpose) computer algebra system. There is support for numeric computations, to arbitrary precision, symbolic computation and visualization'. There are more than 10 alternatives to Yacas for a variety of platforms, including Windows, Linux, Mac, Online / Web-based and Android. The best alternative is Wolfram Alpha, which is free. Other great apps like Yacas are Mathematica (Paid), Julia (Free, Open Source), SageMath (Free, Open Source) and Maxima (Free, Open Source).
Wolfram Alpha is a "Computational Knowledge Engine" - different from a search engine, it answers factual queries directly by computing the answer from structured data, rather than providing a list of documents or web pages that might contain the answer.
SageMath is a free open-source mathematics software system licensed under the GPL. It builds on top of many existing open-source packages: NumPy, SciPy, matplotlib, Sympy, Maxima, GAP, FLINT, R and many more.
Maxima is a system for the manipulation of symbolic and numerical expressions, including differentiation, integration, Taylor series, Laplace transforms, ordinary differential equations, systems of linear equations, polynomials, and sets, lists, vectors, matrices, and tensors.
wxMaxima is a document based interface for the computer algebra system Maxima. wxMaxima uses wxWidgets and runs natively on Windows, X11 and Mac OS X. wxMaxima provides menus and dialogs for many common maxima commands, autocompletion, inline plots and simple animations.
Calcpad is a lightweight mathematical platform for cloud and desktop. It parses simple text scripts and converts them into professional Html reports. Supports complex numbers, variables, functions, graphing, units and numerical methods.
Axiom is a general purpose Computer Algebra system. It is useful for research and development of mathematical algorithms. It defines a strongly typed, mathematically correct type hierarchy. It has a programming language and a built-in compiler.