If you can't access Gmail, try some of these URLs:
Safe mode - http://mail.google.com/mail/?labs=0. It disables the experimental features from Gmail Labs, just in case some of them are buggy. You can remove some of the features from Gmail's settings page.
Secure mode - https://mail.google.com/. It encrypts the traffic between your computer and Gmail's servers. Use it from public computers, Wi-Fi networks or to bypass some proxies and web accelerators. There's a Gmail setting that redirects the standard version to the secure mode ("Always use https").
Older version - http://mail.google.com/mail/?ui=1. This version has been replaced in October 2007 by a rearchitectured Gmail, but the old version is a little bit faster.
iGoogle gadget - http://www.google.com/ig/gmailmax. This is the canvas view for the updated Gmail gadget which can be found in the new iGoogle. Some people found that this interface bypasses most corporate filters that prevent them from accessing Gmail at work.