CMB2 is a developer's toolkit for building metaboxes, custom fields, and forms for WordPress that will blow your mind.

Redux is a simple, truly extensible options framework for WordPress themes and plugins.

A Simple and Lightweight WordPress Option Framework for Themes and Plugins

~ Lightweight, Flexible & Rapid WP Development Framework ~

A Composer package that provides metabox orchestration for WordPress.

Create custom fields with simple and easy to use WordPress meta box framework.

Simple, Lightweight & Flexible WordPress Settings Framework

Clone Of CodeStar but with more powerfull and reliable

A PHP class that facilitates the creation of custom WordPress metaboxes

WordPress metaboxes for themes and plugins.

Ζεύς — Olympus Zeus is a framework which allows you to make all your WordPress plugins and themes developments easier and efficient.

A Custom metabox framework for WordPress to quickly generate metaboxes

A WordPress plugin framework.

WordPress Think Framework is a developer-oriented library that gives you the ability to quickly and easily create incredible websites with WordPress. This is an extensible framework for developing WordPress themes. Based on the WordPress API.

Provides a compatible solution for some personalized themes that require custom meta boxes for WordPress.

AbuFramework is Options Framework for WordPress themes and plugins.

pix_builder

Visible metaboxes for the block editor: add some contrast and margins around Gutenberg metaboxes.

Display SlideShare thumbs as featured image in Wordpress

Just drop the admin folder into your custom WP theme and include the init.php file from your function.php, and you have access to a full range of admin option fields and metaboxes.

Extending Butterbean, a neat little post meta framework by @justintadlock, with custom Settings and Controls

WordPress plugin to Logus theme