For a long time I was developing websites using MAMP and creating separate installations of Drupal in sub directories of my htdocs folder to allow me to work on several different projects at a time. This works well except for one thing. The URLs of the different websites include the sub directory in their name (for example http://localhost/drupal). I had wanted to find a way to configure MAMP to allow me to map these sub directories to sub domains, but I found an easier/quicker way to accomplish the same goal.

In the MAMP Settings → Preferences you can choose the directory that MAMP uses as the root directory. By default it uses the htdocs directory, but you can switch this to any directory you want.

MAMP Apache Settings