MAMP, packaged by webEdition, looks to be quite useful:
MAMP installs a local server enviroment in a matter of seconds on your Mac OS X computer, be it PowerBook or iMac. Like similar packages from the Windows- and Linux-world, MAMP comes free of charge.
MAMP is installed in the typical Mac fashion: very easily. MAMP will not compromise any existing Apache installation already running with your OS X. You can install Apache, PHP and MySQL without starting a script or having to change any configuration files!