When I was developing on a PC, I used a program called PostCast (http://www.postcastserver.com/) to act as my local SMTP server and catch any messages that my web applications generated. It then had a GUI where I could view the messages as if I were on the receiving end. The server could also be configured to never actually send the messages but hold them indefinitely until I manually removed them. This worked out great especially when I was testing against real customer data.
I've since moved on to a Mac and I've yet to find a good way of doing the same thing. For now, I've just been manually changing all of the email addresses to my own and viewing them in Gmail, but I would prefer to be able to mimic the functionality I had in PostCast. Is there a similar way of doing this on Snow Leopard? (I've currently got PHP configured to use the built-in postfix mail server. FWIW, I don't use any desktop mail applications [e.g. Mail.app] and shouldn't really ever need to actually send email directly from my machine.)