The Amazon Kindle, first of its name. When the original Kindle launched in 2007, it was immensely popular, selling out within hours and remaining out of stock for months, but it also wasn't the eReader juggernaut that the Kindle is today. It had a 6-inch screen and 250MB of memory, which was expandable by SD card, and originally went for $400.
The Kindle 2 wasn't a huge step forward, but it's sort of when the Kindle started to take shape as the ass-kicking eReader that we think of today. It ditched the asymmetric design and weird diagonal split keyboard of the original, and added a rough text-to-speech option. It launched at $360
The distant cousin that you see twice a year and who everyone forgets exists. The Kindle DX has a 9.7-inch screen, 2.5 times huger than the regular Kindle's screen, but it's regularly forgotten in lieu of its smaller, cheaper kin. Which is a shame, since the DX was actually a step forward technologically and conceptually that the Kindle 2 wasn't. The original had native PDF support and an inclinometer, and the larger screen was supposed to pave the way for newspapers and textbooks to find their way to eReaders.
There's a lot of unfulfilled promise to the Kindle DX, and the advent of the iPad and tablets in general stole some of its thunder, no doubt, but its prohibitive price point is probably what killed it. It was originally $490, now $380, which is kind of nuts when compared to current tablets and the cheaper Kindle models.
With the Kindle 3 and its $140 price point, Amazon basically took ebooks from the realm of the technological bourgeois to the point where if you read and didn't actively despise technology, you really had no excuse to not own one. It came in Wi-Fi only and 3G ($190) flavors, and a sharper pearl screen and a pretty graphite body.
The $114 Kindle with Special Offers is exactly the same as the Kindle 3, just cheaper and with ads. It's especially notable as precedent for Amazon taking every step it could to push the price down on its hardware. Which leads us to...
Amazon's new Kindle Touch is $99 and the non-touch model is $79. That's totally nuts and puts them squarely in impulse-buy territory. The line also got a refreshed design, with a nice grey matte finish, and dropped the maligned-but-functional keyboard to trim down the size. They're basically everything you could possibly ask of an E Ink reader short of a color display or an E Ink/LCD hybrid—both of which Amazon's already working on.