http://kohanaframework.org/
A supposedly fast PHP framework, at least fast to build. Don't know much else.
http://fuelphp.com/
A PHP framework, meant to be fast to build. Don't know much. Hat tip Richard Flett.
http://www.spoon-library.com/
A PHP5 library that claims to be fast, easy to learn and fun. By the folks who make a twitter PHP class.
http://www.symfony-project.org/
symfony is a powerful PHP framework. It has some very impressive scaffolding (CRUD interface) tools, is highly customisable, and designed for enterprise level programming. It looks good for crud related projects, perhaps not so much custom projects.
http://www.sitepoint.com/article/developing-facebook-platform
An article from Sitepoint.com on how to develop PHP apps for Facebook