pyMaxima
CAS in Python using Maxima & PyGeo

pyMaxima is a simple to use GUI for Maxima, written in Python

"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." (from http://maxima.sourceforge.net/)
The Maxima computer algebra system was started by William Schelter, the late maintainer of Maxima. 
Try also wxMaxima: wxMaxima "is a cross platform GUI for the computer algebra system maxima based on wxWidgets." (from http://wxmaxima.sourceforge.net/)

"PyGeo is a framework for the creation of dynamic geometric constructions" (from http://pygeo.sourceforge.net/). PyGeo's creator was Arthur Siegel
About . . .

Features (EN)

Installation (EN)

pyMaxima Lizenz (GPL)
Download
 pyMaxima.zip (zipped, version 1.80, 20160713, 380 KB)
pyMaxima is tested under Linux, Windows XP and Windows 7.
PyGeo
pyMaxima uses pyGeo for the 3Drepresentation of the geometry objects as lines, planes or spheres: pyGeoexample. Note: pyGeo is using additional resources:

Numerical Python (Numerical Python is bundled with VPython)

Note: The installation of PyGeo is limited up to Python 2.7
You have to install the following package, if you are using Python 2.5 to 2.7:
Many Thanks to the blogger for the update of PyGeo!
pyMaxima will work without pyGeo, but you won't see those funny litte pictures :)
MaximaDokumentation

Wiki (MaximaDocumentation, german)
Note
"Be warned that these systems are no substitute for hands on work with equations and struggling with concepts. These systems do not build your mathematical intuition, nor will they strengthen your core skills." And: "Do not use a computer as a substitute for your basic education." From The Maxima Book, S. 7
Credits
Many Thanks to

The folks, who are maintaining Maxima.

Andrej Vodopivec for wxMaxima, it's great!

James Amundson for his interfaceexample.lisp

Jarno Elonen for his GaussJordanElimination

Josiah Carlson for his Module to allow Asynchronous subprocess use on Windows and Posix platforms (Python Recipe 440554 from the Python Cookbook).

Wolfgang Urban for his statistical distributions

Greg McFarlane and Peter Munnings for the Python megawidgets

Arthur Siegel for PyGeo

The unknown japanese blogger for the update of PyGeo

Daniele C. and many more for Wiki on a Stick

Guido van Rossum for Python