MAMP [mamp.info] is definitely an option although I don't personally like needing to have an open application in the dock for my dev sites to be accessible.
Versions of Apache2 and PHP5 are already installed on current shipping Macs, however Apache 1.3.33 and PHP 4.x are enabled by default. Adjusting your configuration to use the newer versions isn't too complex although it does require editing config files.
Or, you could install newer versions of Apache2 and PHP5 using one of the easy installers which are available:
PHP 5 [entropy.ch] from Marc Liyanage Apache2 [serverlogistics.com] from Server Logistics