While the Sony and Kobo e-readers are not among the most popular, I have found a site where owners of these two can download free e-books!
The site is Inkmesh. In addition to Sony and Kobo, they also list free books from Smashwords, Kindle, B&N, and Baen. Searches can be made by title, author, genre, or e-reader. They also list books that are not free but that can be helpful when searching for a particular author/title.
Inkmesh can also be found on Twitter at @inkmesh. I frequently use Twitter for finding free e-books as the tweets automatically appear on my Twitter homepage and I don't actually have to go looking for them. Yes, I know, incredibly lazy but it works!!
So head over to Inkmesh and download, download, download!!!