Saving the Planet & Stuff eBook is now available for Kobo, as well as Nook and Kindle. All three of my eBooks published by G.P. Putnam's Sons, Happy Kid!, A Girl, a Boy, and a Monster Cat, and A Girl, a Boy, and Three Robbers, are also available in all three formats.
While Kobo isn't as well known as Kindle and Nook, it is supposed to account for 20% of the e-reader market. In addition, Kobo has a relationship with the American Booksellers Association, which involves independent booksellers selling both the Kobo readers and eBooks.
Thus making their eBooks available on Kobo gives self-published writers an opportunity to maintain some kind of connection with independent booksellers.