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.
Basic mode - http://mail.google.com/mail/?ui=html. It's the version that doesn't use JavaScript, so it loads faster and it works well with older browsers. Unfortunately, many Gmail features are missing (contacts autocomplete, chat, spell checker, rich formatting) and each click loads a new page. If you like this version, click on "Set basic HTML as default view" at the top of the page.
Mobile mode - http://mail.google.com/mail/?ui=mobile or http://m.gmail.com. This is a simplified Gmail interface for mobile phones that has even less feature than the basic mode. Use it if no other Gmail mode works for you.
iPhone mode - http://mail.google.com/mail/x/gdlakb-/gp/. A more user-friendly mobile version for iPhone and other mobile phones that use WebKit-based browsers.
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.
"No browser checking" mode - http://mail.google.com/mail?nocheckbrowser. If you use a cutting-edge new browser and Gmail serves you the basic HTML mode, try this URL to bypass browser detection.