Currently the only method to access Gmail aside from the main web interface is through POP3 access. There are some serious drawbacks, including the fact that POP access requires the downloading of each email message, and also that with Gmail, you can only access it from one POP client. There is no 'leave message on server' option, so after an email is accessed once, it disappears from the POP access server.
However, I stumbled upon a very cool PHP class that allows you to access Gmail with PHP and XML. It does this by pulling and parsing the RSS feed from your Gmail inbox. Here's a link to the PHP class.