If you are an Ember Data user, I have created a specific renderer that output JSON compliant payload. I didn't released it yet as it's not really clean, but if you're interested, please contact me :).
Install the module by typing (or add it to your composer.json file):
php composer.phar require zfr/zfr-rest:0.3.*
Then, add the keys "ZfrRest" to your modules list in application.config.php file, and copy-paste the file
zfr_rest.global.php.dist into your autoload folder (don't forget to remove the .dist extension at the end!).
Apigility is a Zend Framework 2 API builder that also aims to simplify the creation of REST APIs.
ZfrRest and Apigility philosophies are completely different. ZfrRest is Doctrine only, and focuses only on a very small subset on your REST API: it provides routing, validation and hydration.
On the other hand, Apigility comes with a graphical user interface, versioning support, authorization, authentication HAL, content negotiation... ZfrRest will never provide all those functionalities, so if you need them, just go with Apigility.
ZfrRest's scope is much more limited (although I'd really like to add support for versioning and links), but I really think it's a nice product too. So give it a try to both products, and choose the one you prefer!
The official documentation is available in the /docs folder.