You might look at sites such as hotscripts.com and notonebit.com to see what they have to offer. An alternative might be to temporarily install a copy of a discussion board (e.g., SMF) or an ecommerce site (e.g., osCommerce) from cPanel > Fantastico scripts and study how they do logins and manage sessions. You want to keep logged in as you go from page to page, plus automatically log the visitor out if they forget to do it themselves (timeout on inactivity on your site). And you want to do it all securely, with no one able to grab passwords. Another alternative might be to take your current site activities and plop them down within  a Wiki, e-store, discussion board, or whatever, taking advantage of their ability to log users in and out, and making your code a subsystem of them. Adding a robust log-in system is not trivial, but you don't have to invent it from scratch.