Full credit for this command line fu goes to Kevin Goodman and his article with the same name. I highly suggest you read his blog post as I don't feel comfortable copy/pasting some one elses hard work.
The command is:
cat /dev/random | tr -dc 'a-zA-Z0-9' | fold -w 16 | head -n 20
The characters in the tr command can be changed to include/exclude allowed characters in the generated password.
The '16' in the fold command can be changed to increase/decrease the length each generated password.
The '20' in the head command can be changed to increase/decrease the number of generated passwords.