MAMP (Macintosh, Apache, Mysql and PHP) is a straightforward tool which makes it easy to install a complete web testing environment on your own computer.
The package includes the same industry-standard software you'd get on a regular server: Apache, MySQL, PHP, Python, Perl and more.
Despite this, you don't need to be a web expert to get started. There's no need to edit configuration files or read up on Apache directives, the installer sets up everything all on its own.
The tiny console shows just how simple this can be. There are only 4 major actions: "Open Start Page" shows you a hosted page at http://localhost/MAMP with details about the program; "Preferences" lists key MAMP settings; "Stop Servers" turns the servers off when you're done, and "Quit" closes the program down.
Even these options can mostly be ignored. Just save your test site to MAMP\HTDOCS, enter LOCALHOST in a browser and it'll immediately be displayed. You're then able to tweak and retest it as often as you like, without needing to use public server, or upload and download your source code each time.
MAMP Pro is a $59 version of the package with many extra features. There's a tester to see how your site looks on iOS devices; multiple PHP version support; full Nginx support; multiple virtual server support; one-click install for Wordpress, Joomla, Drupal and other extras; in-app updates; dynamic DNS support; a built-in code editor and more.