Here is a new tutorial from http://www.bsdvault.net for all you BSD users.
It shows you how to change them and why.
Also shows a quick explanation of their weaknesses.

Changing the Default Password Encryption Algorithm
http://bsdvault.net/sections.php?op=...ticle&artid=89

Here is another link from the same page explaining the different encryptions methods.
http://security.tao.ca/crypt.shtml