I have been using Xdebug for a while now during development of my PHP projects.
The feature that I have found most useful is its ability to create cachegrind files which can then be interpreted by tools such as KCacheGrind.
Cachegrind files contain all the function calls made in you PHP application – this can contain invaluable information which can help you streamline parts of your code which are unnecessarily wasteful. As an object-oriented programmer you sometimes get your head up in the clouds and need a bump down to earth as to how your code is going to perform in real life, kcachegrind helped me to identify some hard to find bottlenecks in my code.
Oh, and did I mention that it automatically dumps pretty stack traces?
