Six to Five
Another one from the old site, this one was submitted by Ed van der Salm
sed -i 's/:$6/:$5/g' /etc/shadow
What it does
It changes all entries in
/etc/shadow that use a sha256 hash to use a md5 hash, without actually changing the hash itself.
Why it works
Md5 hashes are not sha256 hashes, causing all logins using a password to fail.