This Diceware Passphrase Generator uses a number of rolls of a die to generate a very strong passphrase for you. Arnold G. Reinhold asserts on his Diceware Passphrase FAQ, quote:

• Four words are breakable with a hundred or so PCs.
• Five words are only breakable by an organization with a large budget.
• Six words appear unbreakable for the near future, though they may be within the range of large governments.
• Seven words and longer are unbreakable with any known technology.
• Eight words should be completely secure for some time to come.
  

One set of five rolls will generate a single word for your passphrase.