Works for Snow Leopard too! Click here for the original source.
Step 1 – Open the Terminal.app
Step 2 – Open the hosts file
Open the hosts by typing on the Terminal that you have just opened:
$ sudo nano /private/etc/hosts
Type your user password when prompted.
Step 3 – Edit the hosts file
Simply append your new mappings underneath the default ones.
You can navigate the file using the arrow keys.
Step 4 – Save the hosts file
When done editing the hosts file, press control-o to save the file.
Press enter on the filename prompt, and control-x to exit the editor.
Step 5 – Flush the DNS cache
On Leopard you can issue a simple Terminal command to flush the DNS cache, and have your host file changes to take immediate effect:
$ dscacheutil -flushcache
You can now test your new mapping on the browser!