I recently completed a very cool project where I had the pleasure of working with Matt Ranlett. A big element of the project is a membership provider that Matt built that uses a SharePoint list as the user store. The client, Preparis, has very graciously allowed Matt to put the result of on CodePlex here.

Project Description
This project contains a custom Membership and Role provider for SharePoint 2007 which allows you to store your users in a SharePoint list on the site. This gives you the advantage of being able to maintain your users via the simple SharePoint list UI without requiring custom SQL access web parts

Custom SharePoint 2007 List-based Membership and Role Provider

This custom SharePoint 2007 forms based authentication provider was created to fit the following customer requirements:

• on a single server (farm), use forms based authentication to validate external users to prevent the need for Active Directory accounts
• on the single server (farm), host 20 to 30 or more site collections, each exclusively relying on forms based authentication user validation
• each SharePoint site collection must maintain a private set of users
• grant the ability to independently back up and restore each individual site collection, including the user base
• include easy to use and cheap to build membership list maintenance tools

Here is a link to Matt's blog post on the topic. Congratulations Matt!