GeeMail is a cross platform desktop Gmail client based on Adobe AIR which gives you a simple way to access your Gmail account, whether on or offline.
It requires Adobe AIR to run (which is why it can run on Mac and Linux too), and opens as a very simple window with a simplified Gmail looking interface. On first log in, it can take some time for GeeMail to download your inbox, but after that it's as fast as Gmail in your browser. The advantage of having everything loaded onto your Hardrive means you can access your inbox and write email offline (which is sent as soon as you go online again).
There are absolutely no options in GeeMail, which is a shame, as your attachments are not downloaded with your messages, so you can't access them offline. While it looks like a stripped down Gmail, this is actually disappointing, as Adobe Air has given us some of the coolest looking desktop apps of recent times, and it would have been nice if GeeMail was a bit more creative.
If a very simple Gmail client is what you are looking for, this is fine, as it requires absolutely no setup. However, there is competition in the form of Gmail Lab's own Offline, which gives you fully synchronized offline Gmail access.
GeeMail is capable, but far too simplistic and in dire need of more options.