If you need to generate a secure passphrase, an easy and secure method is to use dice. Diceware has a nice description of how to do that. It uses a large list of words and a set of dice. You shake the dice and look up the word that was selected.

This page will help do those lookups faster. It is written in JavaScript and runs entirely in your browser – no information is relayed back to my server. Also, it can generate die rolls for you, but I agree with Arnold Reinhold and say that this software really shouldn't generate passwords for you. There is no good way for me to prove that the random number generator in JavaScript is of a high enough quality to ensure that the words it picks are random enough. That said, I would say it is good enough for most people.

When you get done generating your passphrase, you might want to see how strong it is.