In a new post to his blog Lukas Smith has a list of PHP projects that have popped up around the Symfony2 framework and are following some of the framework's standards for development.
Here is a short overview in alphabetical order of libraries that have spawned around Symfony2 (and just to clarify not at all focused on just Symfony2). Common to all of them are that they require PHP 5.3 and follow the PSR-0 for class loading.
Among the tools mentioned are things like: