From Ebook Friendly:
Free ebooks are not very well highlighted on Kobo (it’s actually the problem with most ebookstores). There are over 1 million free ebooks here, most of them are classic literature from public domain. To browse for free ebooks, simply perform any search and then from a drop down list select Free Only. You can further sort titles by ratings, title, or publication date.
Alternatively, you can use a landing page for free Kobo ebooks. The link is kobobooks.com/free_ebooks. From here you will be able to check top free books in six categories, including romance, mysteries, and classics.
Link to the rest at Ebook Friendly