Another commonly used server, for Mac OS X, is MAMP, which is a distinct project from WAMP or XAMPP.
The pros to these stacks are a one click install for everything you need. For example, Mac OS X comes with PHP, however it lacks most of the facilities necessary to build a web application. Compiling and configuring PHP or MySQL on Mac OS X is a little more complicated than some web developers want to get for something as trivial as setting up a web server. Configuring path settings and Apache to use your new PHP installation is a little beyond many people, such as web designers, so MAMP, XAMPP, and WAMP instead sandbox everything they need in one location. While anyone who has set up a web server on Linux will know to look for configuration files in /etc, data files in /usr, and so on, neither Windows or Mac OS X (on the surface, anyway) use that filesystem hierarchy so it just doesn’t make sense to a lot of people.
Note that if your server stack doesn’t include a built in wizard or something of the like for configuring your virtual hosts, it might take a bit of fiddling with Apache configuration files (I loathed that when I started learning Apache) for you to get the same functionality out of it as you did with PWS/IIS.
@picardo I think Acquia equates to a very specialized framework for setting up Drupal sites quickly. While convenient, it’s not as generalized as a simple server stack. It’s more along the lines of a web framework—quick configuration, built-in web server—anything to get you staring at a freshly installed site ASAP.