Diceware lets you generate random passphrases by rolling dice
You are now expected to roll the five dice again and enter the new result. Repeat this process for as many random words as you wish to have on your passphrase. If you wish to have no spaces, click "collapse," and the tool will remove all of the spaces between the words.
While I find this interesting as an experiment, I would advise against using it for anything serious since the passwords it generates are cryptographically weak. They contain no uppercase letters, no digits, no symbols, and they are entirely composed of dictionary words from a limited, publicly available list (that is right on the tool page).
If Iain Lamb added random capitalization, a digit separating each word, and a couple of special symbols surrounding the password, that would go a long way to providing a truly secure passphrase. Until that time, Diceware remains an interesting experiment in generating random passwords/phrases.