Simplicity is a lightweight modular Web development framework. It is designed with a tight core providing centralized configuration, caching, auto loading, session handling, and a simple ORM. All other functionality is provided by modules which can be loaded in module chains. Each application can have multiple chains.
Kohana is a PHP 5 framework that uses the Model View Controller architectural pattern. It aims to be secure, lightweight, and easy to use. Although it reuses many common design patterns and concepts, there are some things that make Kohana stand out. It is community driven, uses strict PHP 5 OOP to provide visibility protection, automatic class loading, overloading, interfaces, abstracts, and singletons, and is extremely lightweight. Kohana has no dependencies on PECL extensions or PEAR libraries. Large, monolithic libraries are avoided in favor of optimized solutions.
Mouf is a special kind of PHP framework. It combines an Inversion of control (IOC) framework and a component download system in a nice Web-based user interface. The goal of Mouf is to help you use and re-use components. By itself, it does not provide anything useful, but it will help you download and install libraries of components, and bind those components together. You will get better and cleaner code using Mouf. Mouf features more than 100 packages providing various ready-to-use components of a Web application, ranging from an MVC framework to an ORM layer, logging, form generation, image manipulation, Drupal integration, authentication, etc.