Working on multiple Web Projects on the same system might become a headache to manage. You must use Virtual Hosts if you are using Apache as pointed out by David Kiss. Following are the steps required to create a Virtual Host called magento in your system such that when you type http://magento the site is displayed, as per Mac OS X Lion using MAMP:

1. Backup your files.
   1. Open a terminal window.
   2. Backup your hosts file:
      sudo cp /private/etc/hosts /private/etc/hosts.bak
   3. Backup your Apache configuration file:
      cp /Applications/MAMP/conf/apache/httpd.conf /Applications/MAMP/conf/apache/httpd.conf.bak
   4. Backup your Virtual Hosts configuration file:
      cp /Applications/MAMP/conf/apache/extra/httpd-vhosts.conf /Applications/MAMP/conf/apache/extra/httpd-vhosts.conf.bak
2. Update the hosts file.
   1. Edit the hosts file:
      sudo nano /private/etc/hosts
   2. Add the name of the Virtual Host you want on your system:
      127.0.0.1 magento
   3. Save and exit:
      Press Ctrl+X then Y.
3. Update the Apache configuration file.
   1. Edit the Apache configuration file:
      nano /Applications/MAMP/conf/apache/httpd.conf
   2. Un-comment the line where httpd-vhosts.conf is found such that it looks like this:
      Include /Applications/MAMP/conf/apache/extra/httpd-vhosts.conf
   3. Save and exit:
      Press Ctrl-W then Y
4. Update the Virtual Hosts configuration file.
   1. Edit the configuration file:
      nano /Applications/MAMP/conf/apache/extra/httpd-vhosts.conf
   2. At the end add the following:
      <VirtualHost *:80>
      ServerName magento
      DocumentRoot /Users/fraki/Sites/magento
      </VirtualHost>
   3. Save and exit:
      Press Ctrl-X then Y
5. Place the site contents in the folder specified.
6. Navigate to the virtual host.

You might want to check the official documentation of Apache about Virtual Hosts, in this post I am using name-based virtual hosts.