Scientific Python

Distributions

Anaconda from Continuum Analytics.

Python distribution focused on data analysis and visualization
Available for Win (32/64 bit), Mac (64 bit) Linux (32/64 bit).
Supplies 100+ packages and its own command line package manager called conda
Completely separate install from system version
Easy to create virtual environments with Python 2.6, 2.7, or 3.3 for specific projects
Base distribution is free
Paid addons (free for academic use) help speed up using Intel's MKL library and compiling code to be run on GPUs that support CUDA

Canopy Win/Mac/Linux

SciPy Superpack Mac (current only)