In the SciFi category, there's
David Weber's
Honor Harrington novels. The first one,
On Basilisk Station is available at
webscription.net/free/ for free in a variety of formats. (As is
Changer of Worlds, but that book should be read
much later in the series.)
I'd also like to repeat the reccomendation of Orson Scott Card. He writes some of the best SciFi and Fantasty books out there. Look for his Ender Trilogy,
Enchantment, and the Homecoming trilogy.
Hamishmacdonald mentioned Cory Doctrow—the free ebook he released,
Down and Out in the Magic Kingdom, is just about the weirdest book I've ever read. It deserves awards for sheer creativity.