WordPress.org

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/plugins/slt-custom-fields/docs/.

Code on GitHub.

Issue tracking on GitHub. If you're not sure if you've found a genuine issue or not, please start a thread on the WP forum.

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!