Ereaders that support epub files, like the Kobo, allow you to use downloaded books, including lots of free books from Google. No, this doesn't work with a Kindle, which doesn't support epubs.
I use Adobe Digital Editions to manage my epub ebooks. It is free software from Adobe. It requires an Adobe ID to get and it means that the books you download, or buy, are on your computer, not just on your ereader. Call me old-fashioned, I like this.
I started reading books on my old palm pilot many years ago and have only had the Kobo for a few months, but I really like it. I love the idea of having lots of books stored and at my fingertips. I'm not crazy about being obligated to buy them from a particular vendor, so I really like the idea of the epub file format. You can also read epub files on Apple iPads, Androids, Nooks and a Sony Readers.