cURL is a command line tool for doing all sorts of interesting and essential URL manipulations and data transfers.

Cheat Sheet

Check gmail

Check HTTP Headers

Get a FTP directory listing

Get a HTTP Basic authenticated page

Perform imap queries using Curl

Post to a page

Read a HTTP URL

Read a HTTPS URL

Read an IP address

Reading a URL with variable GET parameters

Read a web page and store it in a file

Referer & User Agent

Track Web Redirects

Upload a file with FTP