Based in part on over 3,000 newsgroup articles written by Python veteran Fredrik Lundh since 1995, this book provides brief descriptions and sample scripts for all standard modules in the Python 2.0 library.
For more information on the book and the print editions, see (the eff-bot guide to) The Standard Python Library.
Note that the book was written for Python 2.0, but most of the code still works in current versions.
You can get the chapters in PDF form here.