What all are best sites for free downloading books?

Use a warez site and don't ask these questions again. When ACTA is enforced you will end up in jail for questions like this. So please for your own good get used to stealthily downloading your stuff and not asking anymore.

If you like to read, then please support the writers by not downloading from sites that pirate books. There are plenty of good sites on the net with a great assortment of free books. The one I use the most is Feedbooks. The site is divided into public domain books and books that are posted by people sharing their own work. Most of the modern books are amateur work and some of them aren't very good, but published authors also have some of their books available there. Two science fiction writers, Cory Doctorow and Charles Stross are sharing a lot of their work freely. Feedbooks lets you post your reviews or comments about the books you download, and you can have your own bookshelf. It's not a fancy site, but I've found a lot of very good books there.