Maxima and REDUCE are large, free software Lisp programs with a GUI.
For Maxima, the wxMaxima GUI is recommended (included).
Both Computer Algebra Systems can be operated from a command line.
Verify your math with these mature CASes, lots of work went into them.
Maxima was made open source in the year 1998,
and REDUCE became open source on December 18, 2008.
Do symbolic and numeric mathematics online with our public server,
or download the latest free source code or binaries and run it on your computer.
Mathomatic became open source in the year 2000, and is a mature CAS too, though not full-featured, more like different-featured.
New and novice users should find it easy to use and very powerful.
Never forget that Wikipedia is one of the best, concise information sources on planet Earth.
There are two of them, the English
one and the Simple English one.
You are all welcome to help them out, even anonymous users.
It is everyone, working for the good of everyone!