Try
http://www.evolt.org/node/60384.
I'm using it on the JSiVi Beta (
http://www.jsivi.co.cc/beta)
It's from 2004, but it works very well.
And it's free!
And it uses the deprecated eregi_ functions! And it's about as insecure as it can be, given that unsalted MD5 is only very slightly better than storing passwords in plain text! And it unnecessarily restricts values users can use as a password/passphrase! And it is vulnerable to SQL injection attacks!
You might want to take a look at
this thread to see what the potential problems are, and why you need to worry about them even if your site isn't really that important in the grand scheme of things. And despite my criticism of the whole mailing-the-password thing,
callumacrae's authentication script (linked in that thread) is a
hugely better system than the ones pointed to above.