Restler for PHP looks really neat and elegant. It is an open source library that easily exposes your business objects as REST end-points, enabling you to focus on your core business model and allow Restler to take care of your REST API for you, which is exactly the approach that I intend to take when I add REST support to the Alpha Framework. It also reminds me heavily of the Jersey framework for Java, which I have used in the past and found it easy to use for building good quality REST APIs easily and quickly.
Restler supports XML, JSON, and YAML as well as some other formats that I have not played with yet, you can find all of the details on the Restler homepage here:
Updated 2022 : note that the above post was originally published in 2011, but is left here for archival purposes. I have fixed some broken external links above.