PLEASE NOTE: I do not condone the cracking of other people's passwords for anything other than authorised penetration testing and security analysis.

However, if you have 'forgotten' a password for one of your own boxes and still have telnet/SSH access to the box via another account you could try this:

[Summary]
You'll need an offline copy of /etc/passwd and something to generate hashes against a wordlist or brute-force strings.

[Method]
You could 'cat /etc/passwd' and copy/paste from puTTY to (e.g.) Notepad [I assume you're SSH'ing from a Windoze box since you're using puTTY]

You then need something like John The Ripper, a quick machine, and patience.