The PHP::Impact blog has posted a list of thirty classes and components that can make your PHP development life so much easier:

Simplicity and extensibility are the main reasons why PHP became the favourite dynamic language of the Web. In the last decade, PHP has developed from a niche language for adding dynamic functionality to small websites to a powerful tool making strong inroads into large-scale Web systems.

Their list includes software like:

• LINQ for PHP
• Doctrine
• HTML Purifier
• SimpleTest
• Phing
• PHP Security Scanner
• SimplePie
• Smarty

...and many more. Check out the full list for all sorts of useful tools.