If you have WordPress already, you probably saw this in your dashboard. I highlight it here because I’ve been using WordPress as a CMS and site backend, and love to see it used for other things. The developers over at the Design Canopy have put together instructions on using WP as a contact manager. It looks fairly full featured with tagging, searching, relations, and as many custom fields as you need.
WordPress has got to be one of the most versatile FOSS packages today.