[jmoiron]

This is available via the pkg-info in all installed packages; iirc version is required for distutils, and there is pep386 for version number format, so it should be possible to determine version number as well as compare them for all well-behaved packages. There is even a package which will find and parse pkg-info for an installed package called pkginfo:

http://pypi.python.org/pypi/pkginfo

[anthonyb]

GP was talking about built in modules in the standard library. I don't think they use distutils, but many of them still have some sort of version number.