On PHPBuilder.com today, there's a new tutorial spotlighting the use of the cURL library with PHP:
cURL is one of the most powerful PHP extensions. It stands for Client URL, and allows you to communicate with other servers using a wide range of protocols. Perhaps that sounds fairly uninteresting, but give it some more thought.
As food for thought, they give a few examples of the use of this handy library - a script to look up the meaning of a word, how to access password protected pages, getting the information about a file from a remote server, posting to a remote script, and FTPing a file to a remote server. This flexible library allows all of these without making things overly complex and unmanageable.
You can check out the full list of features for the extension on this page of the PHP manual.