XHProf is a hierarchical profiler for PHP. It reports function-level call counts and inclusive and exclusive metrics such as wall (elapsed) time, CPU time and memory usage. A function's profile can be broken down by callers or callees. The raw data collection component is implemented in C as a PHP Zend extension called xhprof. XHProf has a simple HTML based user interface (written in PHP). The browser based UI for viewing profiler results makes it easy to view results or to share results with peers. A callgraph image view is also supported.
So you're sold, but getting it installed in MAMP is kind of a pain.
A little about my setup system first. I'm running Mac OS X 10.6.4 with MAMP 1.9.2 with php 5.2.13 and you'll need Xcode.