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.