I am running a simulation in Python. The simulation's results are summarized in a list of number matrices. Is there a nice export format I can use to write this list, so that later I can read the file in Mathematica easily, and Mathematica will recognize it as a list of matrices automatically?
How big are the matrices?
If they are not too large, the JSON format will work well.  I have used this, it is easy to work with both in Python and Mathematica.
If they are large, I would try HDF5.  I have no experience with writing this from Python, but I know that it can store multiple datasets, thus it can store multiple matrices of different sizes.
Save list of table of numbers from Python into format easily readable by Mathematica?