Symfony2 community pushing the PHP ecosystem
Even if you don't care about Symfony2 or Silex and instead prefer some other framework, there is stuff to benefit from going on inside the Symfony2 community. Not only are the Symfony2 components all build to also work standalone, but key contributors are also building libraries and tools that should proof useful for the entire PHP ecosystem. For an overview of the components part of Symfony2 I refer you all to github. 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.
Behat is actually a github organization that has created a bunch of libraries. The main one is of course Behat itself, which is a BDD framework. But there is also the Gherkin parser, Mink for browser testing and SahiClient for JS test automation.
Buzz is a lightweight HTTP client library.
Imagine provides various tools for image manipulation.
Yet another port of a python library Monolog provides a flexible logging toolchain. One of the coolest loggers is the fingers crossed logger, which collects information in memory but only logs the data in case an error occured.
PS: I guess I could also add Doctrine2 to that list. While the Doctrine2 community is of course much bigger than "just" Symfony2, a lot of contributions have spawned out of the Symfony2 community.
PPS: Maybe the next big contribution to come out of Symfony2 will be composer.