Brad Harris has thrown his hat into the ring with his own (just released) framework, Adriot:

I've been working on this for awhile now, but its finally to a point where I think its something I can share with others. Adroit is something I'm continually working on, and I'd like to share it with the PHP community for others to use and share feedback about it. I've been using this framework for quite some time now as its evolved, and its helped me develop in a more structured and manageable manner. Feel free to check out the doc and source code, and give any feedback on it.

Adroit provides a MVC-style framework that focuses on keeping things lightweight and simple to use (with a smaller learning curve). Key features include the MVC structure, that it's well documented, that it uses the Smarty template engine by default and that it integrates easily with things like Ajax. There's even a demo posted showing what the framework can do.