Has it really been six years? The iPod nano was first made available for purchase on September 7, 2005. At the time it debuted, the iPod mini was (according to Steve Jobs) the most popular MP3 player in the world. Carrying on the tradition of "1,000 songs in your pocket" the nano was an iPod with a clickwheel shrunk down into a case that was thinner than a number 2 pencil. More importantly to the future of Apple, the nano used flash memory instead of a tiny hard drive -- a move that would eventually work its way through almost all of Apple's product line (and will likely continue). Available in black and white, the nano was available in 2 GB and 4 GB models but Apple added a 1 GB version later.
Unfortunately, the first generation nanos were prone to scratches, and Apple had to reach a settlement in a class action lawsuit involving the scratches and a slip case for the device. Later models proved to be more robust, however, made of aluminum. Personally I think the nano taught Apple quite a bit about durability in their products, and it's possible the short time it took for the nano to come to market may have been a part of the problem. Nevertheless, the nano has evolved greatly over the years.
I know people who still use their first-generation nanos to work out (using the case I designed in the image above). Do you still have one or use one? Check out the video of Steve Jobs introducing the iPod nano below.