SymPy is a Python library for symbolic mathematics. It aims to become a full-featured computer algebra system (CAS) while keeping the code as simple as possible in order to be comprehensible and easily extensible. SymPy is written entirely in Python and does not require any external libraries.
via sympy.org
I don't really have a need for symbolic math right now, but it's good to see a pure Python library for this. I much prefer the approach of small libraries with few dependencies, instead of using a hundred megabytes download like Sage (which I use only for the web notebooks, they're great for giving Python courses!).
When I started my PhD I used Maxima for my Geophysical Fluid Dynamics assignments and it was very impressive. Even though symbolic math is simply based on rules on how to manipulate symbols it always impressed me to see a computer integrating an equation symbolically instead of numerically.