The earliest that I still see on the market on occasion is the Kobo Wi-Fi. This was the second Kobo E-reader I got, and as a basic model, it still does absolutely fine. If I remember correctly, it held up to a thousand books and the battery typically lasted about two weeks. I remember getting about three to five books read on it at a time. I know this one was on the market in mid-2011 because I wrote a post about it around then. This was the first Kobo to give more than simply a choice between black and white. There was the black, all-white, white and lavender, white and silver, white and baby blue, and I think, white with a grey backing. Navigation was done via the thumb-pad in the bottom right corner.
The next models on the market were the Kobo Vox and the Kobo Touch. Two completely different e-readers catering to a completely different audience set. The Kobo Touch was the next generation e-ink reader and the first of the new touch-screen readers Kobo released. The biggest additions this brought about were more control over the page appearance - not just font size, but multiple fonts, line spacing and margins were all adjustable. They also added the ability to touch a footnote and have the book take you to that point and then back to your original page again, which in my mind opened up a lot of non-fiction reading options, not to mention the ability to highlight and take notes on what you're reading. Kobo also improved the battery life dramatically. As with the Wi-Fi, the Touch comes in several colour options, including pink, blue and white backs with a white front, and also an all black model.
load on library books for, and another neat feature was being able to install the Amazon Kindle app as well, giving me the best of both worlds - something that has since been made even easier.
As I said with the Vox, I'd recommend this one primarily for those who want to read books that use plenty of colour, such as graphic novels and the like. I can see from the screen layout that this is geared more for multimedia use rather than for strict reading.
The final and newest e-reader in the Kobo family is the Aura HD. I wrote a full post on it here earlier today, so I'll just summarise some of the points briefly here. Rather than the normal screen of 6 inches that the majority of the other e-ink/pearl readers listed here have, this one has a 6.8 inch screen, making it closer to a hardcover in size. Also, the Aura HD has at least twice the storage of the other e-readers at 4 GB of space. Finally, the ComfortLight has been improves somewhat so there's less bleeding at the edge of the screen. The only negative I've found so far is the removal of the "Free Books" category from the Kobo store menu. The books are still there to be found. You just have to know about them to be able to search by name, rather than being able to browse a list.