If you like free ebooks, you probably already know about Project Gutenberg, and maybe a few other places where you can find them. Like PG, ManyBooks.net offers free, public domain books and stories. However, the ManyBooks site offers a few additional improvements that PG doesn't have.
Like PG, you can search by author or title. However, ManyBooks also has their books sorted by genre, so you can focus on the particular types of stories you want to read, even if you're not familiar with the author or title. Crawling through the genre listings is a great way to discover new authors and titles that you may be interested in.
Furthermore, ManyBooks offers additional "meta-information", including a short description of the book, an excerpt from the book, and the Flesch-Kincaid Reading Ease scale, which gives you a relative idea of how easy it is to read the book. Readers can also add their own reviews of the book, so you can know what other people think, or give your own feedback on it.
They also offer the book in over 2 dozens formats, from plain text to epub, or even as an audiobook, if such a version is available.
Last, but not least, if you create an account with ManyBooks, you can create your own "bookshelves" or lists of books. You can create multiple bookshelves and name them whatever you want. This allows you to keep track of books you want to read, or have read, or just to make books easier to find instead of having to search for them again.
And, of course, it's all free, although they appreciate donations to keep the site running. Check it out.
