If you have the new version of Gmail, but you don't like it, there are some ways to go back to the old version. Maybe you don't have a fast Internet connection and Gmail suddenly feels slower, maybe there are too many bugs or you love a Gmail-related extension or Greasemonkey script that suddenly doesn't work.
Gmail provides a link to the old version (http://mail.google.com/mail/?ui=1), but the change is not persistent, so you'll still see the new version the next time you go to Gmail. And even if you bookmark the link, some Gmail-related plug-ins will still not work.
Because the new version is only available in Firefox 2 and Internet Explorer 7, another way to revert to the standard Gmail would be to use another browser, like Opera or Safari. But that's not very convenient or practical.
So what's the best solution? Change Gmail's interface language in the settings from English (US) to another language, like English (UK). You'll lose some features (creating Google Calendar events, PowerPoint viewer) and some names will be different (instead of Trash, you'll see Deleted Items), but these are minor changes.
Gmail's new version will be rolled out to everyone in the coming weeks and will eventually replace the current version, but by the time it reaches everyone, Gmail will probably fix the performance issues and your favorite plug-ins will update their code. A Gmail API would be a much better idea for the future, because every change in Gmail's code can break a plug-in like Gmail Manager or some useful Greasemonkey scripts.
Update: Obviously, this was just a temporary solution and it no longer works, since Gmail 2 is available for English (UK). If you still don't like the new version of Gmail, bookmark http://mail.google.com/mail/?ui=1, but that address won't be available indefinitely either.