GeeMail (download for Windows|Mac) is a standalone Gmail client that runs on Adobe AIR. It looks and feels exactly like Google's Gmail, or at least how it did before the last year. It's big feature is that it lets you view and reply to Gmail messages while offline. Messages are then sent the next time the application detects you have a connection.
Unlike Google's, GeeMail is quite a bit faster. It grabbed 1,500 of my most recent messages in just a couple of minutes. This speed, while nice, comes at the expense of downloading a much smaller portion of your inbox than Gmail does. For instance, the app won't pull down Gmail's labeling structure unless you're online. This means that if offline, attempting to sort messages by label after the initial download simply won't work--even if you've downloaded a portion of messages with that same label.
The app will eventually pull in the entirety of your inbox (except for attachments and photos), but it does this in chunks. When you've left it idle for a little while it starts downloading additional segments.
Search--one of Gmail's most important features--is missing in GeeMail. It's simply not there, which if you're like me and have an unruly inbox with a lot of e-mails, can make the tool relatively useless. I assume later versions will have something that at least lets you dig through the mail it's downloaded. In the meantime you're left sorting by label or trying to remember when you received a message.
Is there any benefit to using this instead of simply linking up your Gmail account to a piece of dedicated software like Outlook Express, Apple Mail, or Thunderbird? I'm not so sure. Those solutions work offline, and grab things like attachments, as well as having their own search and workflow tools.
One thing this app really has going for it though is the feel of the old Gmail, and a faster, leaner way to pull down mail for reading offline. And, if for some reason you can't use Chrome and take advantage of its special application view--which lets you run Gmail as if it was a separate program--this is one of the easier ways to give it that same feeling.