There are many python packages that are useful for astronomy research.

Multi-purpose libraries

all overlapping at some level and at various stages of development.

AstroLib with all kinds of python utilities specifically for astronomy.
esutil - WCS utilities (wcsutil), cosmology calculations (cosmology), coordinate transformations (coords), and heirarchical triangular mesh sky search tools (htm).
Astropysics - An integrated package and framework for common tasks in astronomy, written to be reasonably "pythonic". Includes GUI applications for common tasks.
pyAstroLib

One Function Packages

Also see AstroPython's list of packages.

pyds9 Python access to ds9 (source tarball for install); AstroBetter pyds9 tutorials
healpy for manipulating all-sky binned data (HEALpix framework)
PyMC for doing Markov Chain Monte Carlos
APLpy for plotting astronomical images in World Coordinate Systems.
CosmoloPy for cosmology calculations
ATpy for using and converting between astronomy tables in different formats (FITS, VO, IPAC, SQL)
PyEphem - high-precision astronomy, ephemeris, and date conversions
pySLALIB - library of routines for accurate and reliable positional-astronomy applications. Original documentation
ChiantiPy - package for calculating synthetic spectra from the CHIANTI atomic database for astrophysical spectroscopy.
ATpy - Seamless multi-format table handler
python-montage - Montage wrapper
IDL Save - read IDL save files into python