Thunderbird

From the same stable as the Firefox web browser: Mozilla, Thunderbird is an Email client.

Working perfectly well as a stand-alone email client, it now has a sister project called SunBird which adds calendar facilities. Like Thunderbird, Sunbird can operate stand-alone but there is also a third project called Lightning that aims to tightly integrate the Calendar functionality into Thunderbird.

Like Firefox, the beauty of this product is that it is pretty much platform independent: it runs equally well on Windows, Linux and Macs.

Also apparently, there is no real bird nicknamed a Thunderbird. (See Firefox)