As a sort-of follow-up to my own post praising SAGE as an all-in-one Python distribution including tons of science goodies, I want to also point out that the Enthought Python Distribution is now available in beta form for Mac OS X (and non-beta for linux and windows).
EPD is smaller than SAGE in size and also includes tons of good science stuff. There’s some overlap between the two distributions (for instance, both include the NumPy/SciPy/Matplotlib triad as well as IPython), but EPD is more geared towards observation data analysis and visualization. For instance, EPD includes netcdf and HDF5 libraries, VTK, WxPython, PyTables, PIL and lots of other stuff. And since it comes from Enthought, it also includes the whole Enthought Tools Suite, which are components aimed at making the development of standalone scientific applications easier (for an introduction to ETS see this nifty tutorial by Gael Varoquaux). The Enthought people also seem to my often-misinformed eye more directly involved into NumPy et al.
I’m not necessarily saying this distribution is better than SAGE’s, but if you don’t feel the need for some of SAGE’s extras (e.g. the Mathematica-like notebook approach, or the tons of symbolic math packages and libraries), you might want to take a look at EPD. The installation is painless and everything works as advertised (at least on Mac OS X).