InfoCentral allows you to track your church members -- people, families, and groups. Group people into families, track group membership, run reports, create custom properties, manage granular user rights, etc. Requires PHP and MySQL. InfoCentral is designed for churches, bible studies, religion/religious groups, but could conceivably be adapted to about anything.

UMA (formerly known as Rampart) is a PHP User Management and Authentication system. UMA supports MySQL, completely object-oriented, easy to implement, and browser based management tool. Professional support available.

A common problem for web developers is MS Windows browsers that have cookies disabled. There are session management servers that work around this, but for a small web site, the cost and effort of setup is usually not worthwhile. This script and example show you how to set up simple server-side cookies with a flat file.