Setting a specific dnsentry
You could add a new line in your hosts file (see man hosts(5)). The easiest that I know of would be to open a Terminal and use nano to edit the file. You could type this in your terminal:

sudo nano /etc/hosts


(replace nano by vi if you are more familiar with the latter). Then go at the end of the file and add something like this:

127.0.0.1    mytest.com


Save the file (key combination for nano is (it's a o upper case): Ctrl + O).

The file hosts is read by mDNSResponder which is the local DNS cache on OS X (incl. Snow Leopard). So your browser will be redirected to the IP you set in the file.