Provides developers with powerful and flexible tools for managing post and user custom fields.
This plugin is aimed at plugin and theme developers who want a set of tools that allows them to easily and flexibly define custom fields for all post types, and for user profiles.
Full documentation at http://sltaylor.co.uk/wordpress/developers-custom-fields-docs/.
Code on GitHub.
Please note that this plugin isn't suitable for non-developers. It has been intentionally designed without a user interface for defining fields, and some aspects may be "unfriendly" to anyone not comfortable with hands-on WordPress development.
If you think this plugin doesn't quite suit your needs, there is a comparison chart of similar plugins to find something that will!