It searches for keywords in the Python manuals at
The advantage over pydoc is that it also looks in the C-api manual, which you need when you are writing extension modules.
The script can also be downloaded, and can be configured to search HTML manual pages installed locally. The keyword index is build on the fly. It can also (probably) be easily integrated in your editor/ide to provide context sensitive help.
Mark Hammond has integrated an interface to this keyword search in his cool Mozilla/Netscape sidebar he recently announced: http://starship.python.net/crew/mhammond/mozilla.