From the Straw Dogs blog there's a recent post looking at some of the PHP frameworks out there have the same kind of philosophy behind them as the Sinatra framework for Ruby - a small, light framework that's easy to use.
I'm currently looking for a good PHP framework to do a new project. I recently used Kohana 3 at on a project for a client but I needed something lighter and having used Sinatra and Padrino previously but needing it PHP based I did the next natural step - searched for PHP Sinatra clones.
There's four frameworks that made the list - all in varying states of development: