Sometimes handy for me: if you have information that you've sent in an HTTP POST, and you need to recover it for whatever reason, there's often no easy way to get at it from the browser interface. The browser will send the data again if you reload the page, so:

1. In a terminal, startup netcat as a listener:

  nc -l -p9999

2. In your browser, configure the connection preferences to use manual proxy settings of host "localhost" and port "9999"

3. Hit reload on the browser, and tell it "yup, send the POST data again"

4. Look at the output of netcat

(Don't forget to change your proxy settings back!)