There are many good password generators out there, including the one built into Keychain Access in 10.4. However, my only complaint is that they all require too much mousing around, which can be slow.
So, based on lectrick's comments on this previous hint and man perlfaq5, the following script places a random 12-digit-string (letters and numbers) password on the clipboard. It sits there for 60 seconds, and is then overwritten with a random entry from Emacs spook.lines file:
#!/bin/sh
jot -rc 24 48 123 | grep -m12 \w | rs -g 0 12 | pbcopy
sleep 60
perl -0e 'rand($.) < 1 && ($line = $_) while <>;' -e \
'print $line;' /usr/share/emacs/*/etc/spook.lines | pbcopy
Remember to make the script executable (chmod a+x scriptname). When used with Quicksilver or similar utility, this becomes quick indeed. However, don't then don't forget about the stack in the clipboard, which will show the previously-created password.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2006010202222194