There are a couple of popular sites that list or host submitted Python modules.
The Python Package Index (aka "PyPI") is the current preferred location to list packages. It is integrated with the distutils in recent versions of Python: you can upload distribution files to PyPI so that the code will be available for downloading. See the CheeseShopTutorial or Uploading Packages to the Package Index for more information.
The Python Cookbook, for small useful snippets of code.
There are a few other less useful or older sites, in case you are searching for code and the above didn't help.
Useless Python is indiscriminate, for all bits of code you don't think are worth publishing.
Vaults of Parnassus - a link store for Python modules. You must host files elsewhere, but this is one of the oldest and largest lists of available Python code.
PyCode - an older site where you could share modules. No activity since 2006.
- This wiki - if a module is relatively short
See also: Italian translation at Codice, FAQTs section on module Q&A, Code