How can I authenticate against a non-drupal database? We have a non-Drupal intranet system built on php/mySQL. We're adding a Drupal install to handle some social functions, and would like (if possible) to not duplicate all of our login information. Is there a module (or technique) that would let us authenticate against our existing users table, rather than needing to create separate user records in the new Drupal install?

I did find this, but it's been abandoned for 6 years, so I don't know how relevant it is anymore.

We have not started the new install yet, so either 6 or 7 is possible - if there's a module that would work for only 6, we're perfectly willing to do that.
My suggestion would be to take a look at LDAP integration or Lightweight Directory Access Protocol modules to see how they handle authentication.

My recollection is that they hook_form_alter the user_login form to add a new validation function that calls out to the LDAP server if Drupal authentication fails.  You should be able to do this for your own authentication.  We modeled a custom authentication on this method, but I don't recall all of the details.

Just remember that you want to use the external authentication in addition to and not instead of Drupal authenticaion so you don't lock out any admins if/when your other site is down.