Once Again SENSIO LABS comes with great things for developers [STAND ALONE COMPONENTS FOR PHP]

Symfony Components are stand alone php libraries that can be handy in the development of php. Those are earlier coupled with the symfony and now available separately from the symfony package.

Just have look at those
YAML : PHP library that speaks yaml. YAML framework is the php library which take yaml as string and convert in to php array. All in all it is great product if you want to use it in any of your php project.
it is open source and available at YAML Component of Symfony

Templating : Which is templating engine, that you can use in any of your php project. It is the templating tool which you can use in any kind of templating system. Available at Templating System

And some other components available like EVENT DISPATCHER, DEPENDENCY INJECTION

And Request Handler The flexible micro-kernel for fast frameworks is coming sooner. have a look at
Symfony Components

All in all if you are now interested in symfony's some of the features to implement only. THIS IS THE BEST OF COMPONENTS YOU CAN TRUST. AND YET NOT REQUIRED TO USE THE SYMFONY. It shows the robustness of symfony projects.