The Simple PHP Framework
SPF: A simple framework for developing WordPress plugins.
PHP Simple MVC Framework is a lightweight MVC Framework
A simple MVC framework I made on top of which I built a project at university
Redux is a simple, truly extensible options framework for WordPress themes and plugins.
A Simple and Lightweight WordPress Option Framework for Themes and Plugins
A minimalist framework for managing WordPress plugin or theme settings.
[UNMAINTAINED] A very simple CMS, just put text files in the pages directory and you're off to go! (no Database required, depends on Zend Framework and Spark Web Framework)