PHPXref
CodeIgniter
CodeIgniter is a powerful PHP framework with a very small footprint, built for PHP coders who need a simple and elegant toolkit to create full-featured web applications. If you’re a developer who lives in the real world of shared hosting accounts and clients with deadlines, and if you’re tired of ponderously large and thoroughly undocumented frameworks
WordPress
WordPress is a state-of-the-art publishing platform with a focus on aesthetics, web standards, and usability. WordPress is both free and priceless at the same time.
Symfony
Symfony is a full-stack framework, a library of cohesive classes written in PHP.
It provides an architecture, components and tools for developers to build complex web applications faster. Choosing symfony allows you to release your applications earlier, host and scale them without problem, and maintain them over time with no surprise.
Symfony is based on experience. It does not reinvent the wheel: it uses most of the best practices of web development and integrates some great third-party libraries.
MantisBT
MantisBT is a free popular web-based bugtracking system (feature list). It is written in the PHP scripting language and works with MySQL, MS SQL, and PostgreSQL databases and a webserver. MantisBT has been installed on Windows, Linux, Mac OS, OS/2, and others. Almost any web browser should be able to function as a client. It is released under the terms of the GNU General Public License (GPL).