Some very good website recommendations today and a few other gems.
roundup of free sf/fantasy/horror by Regan Wolfrom is a must see collection if there ever was one. If don't see anything you'd like in that list, look again, your eyes are playing tricks on you.
Free SF Reader is a great site with daily links to free genre fictions in various formats.
At Beam Me Up: Episode 330 - Episode 12 of Jason Kahn's "In Plain Sight" and part 3 of Ed McKeown's "The Dive."
At Escape Pod: "Follow That Cathedral!" by Gareth Owens.
At Pseudopod: "The Long Road to the Sea" by James L. Sutter
At StarShipSofa: "The Emancipation of the Knowledge Robots" by Carl Frederick.
Digital Comics Museum is one of the best sites to some complete comic books for free. Because this is a completely legal site, there are no recent comics. Instead, there are hundreds of classic pre-code comics from the 1940s and 1950s. There are many sci-fi comics and even more horror comics. With a good comic book reader, some look fantastic on Kindle Fires, iPads, and similar devices.
Ask Magazine is a free quarterly magazine and website from NASA. While a bit dry due to its focus on project management and engineering, there are often articles to interest the layperson. For example, the latest issues has an interesting article "Kepler: The Long Road to Other Worlds." The latest issue, number 47, is available online, and in PDF and epub downloads. Most prior issues are available online and in PDF Downloads. Worth a look.