This command will read 64 bytes of random data from /dev/urandom, base64 encode it, add a small salt (the current data in epoch time), and then create a sha1 hash of the data.
I like this because it's cryptographically secure and the chance of a collision (provided your PNRG isn't totally borked), is infinitely small. It's also a hexadecimal string, so I don't have to worry about quoting it in weird ways or escaping special characters. I can just double-click it in iterm and it's automatically added to my clipboard!
Go ahead and double click the shas below and then click the password from 1password. You'll know what I'm talking about.
The drawback being you can't possibly remember these passwords unless you're US memory champion Nelson Dellis, but you use a password manager anyway, right? Right?!
I do this so frequently that I created an alias, so I just have to type "pw" on the command line to get a random password.