Sumatra PDF supports reading eBooks in ePub and Mobi (Kindle) format. Here’s a list of website that offer eBooks in those formats for a free download.

Standard ebooks has free, public-domain, carefully produced ebooks.

Feedbooks offers both paid and free ebooks.

Internet Archive has lots of free texts available in ePub and mobi (Kindle) formats.

Project Gutenberg has thousands of free, public domain books that are available for download in ePub and mobi (Kindle) format.

Baen is a publisher of science fiction books which offers some of their books as a free download in ePub and mobi (Kindle) format.

Smashwords is an eBook publishers and some of their books are free. They are offered for download in ePub and mobi (Kindle) formats.

The Online Books Page is an index of over 3 million public domain books.